ENH: Implement CSV reading for BooleanArray
#### Code Sample, a copy-pastable example if possible

In 1.0.0 rc0, `pd.read_csv` returns this error when reading NA with specifying dtype of 'boolean'

```python
# import pandas as pd
# import io

>>> txt = """
X1,X2,X3
1,a,True
2,b,False
NA,NA,NA
"""
>>> df1 = pd.read_csv(io.StringIO(txt), dtype={'X3': 'boolean'}) # `pd.BooleanDtype()` also fails
raceback (most recent call last):
  File "pandas/_libs/parsers.pyx", line 1191, in pandas._libs.parsers.TextReader._convert_with_dtype
  File "/usr/local/lib/python3.7/site-packages/pandas/core/arrays/base.py", line 232, in _from_sequence_of_strings
    raise AbstractMethodError(cls)
pandas.errors.AbstractMethodError: This method must be defined in the concrete class type

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/site-packages/pandas/io/parsers.py", line 676, in parser_f
    return _read(filepath_or_buffer, kwds)
  File "/usr/local/lib/python3.7/site-packages/pandas/io/parsers.py", line 454, in _read
    data = parser.read(nrows)
  File "/usr/local/lib/python3.7/site-packages/pandas/io/parsers.py", line 1133, in read
    ret = self._engine.read(nrows)
  File "/usr/local/lib/python3.7/site-packages/pandas/io/parsers.py", line 2037, in read
    data = self._reader.read(nrows)
  File "pandas/_libs/parsers.pyx", line 859, in pandas._libs.parsers.TextReader.read
  File "pandas/_libs/parsers.pyx", line 874, in pandas._libs.parsers.TextReader._read_low_memory
  File "pandas/_libs/parsers.pyx", line 951, in pandas._libs.parsers.TextReader._read_rows
  File "pandas/_libs/parsers.pyx", line 1083, in pandas._libs.parsers.TextReader._convert_column_data
  File "pandas/_libs/parsers.pyx", line 1114, in pandas._libs.parsers.TextReader._convert_tokens
  File "pandas/_libs/parsers.pyx", line 1194, in pandas._libs.parsers.TextReader._convert_with_dtype
NotImplementedError: Extension Array: <class 'pandas.core.arrays.boolean.BooleanArray'> must implement _from_sequence_of_strings in order to be used in parser methods
```

while "Int64" and "string" with NA can be correctly recognized.

```python
>>> df2 = pd.read_csv(io.StringIO(txt), dtype={'X1': 'Int64', 'X2': 'string'})
>>> df2
     X1    X2     X3
0     1     a   True
1     2     b  False
2  <NA>  <NA>    NaN
>>> df2.dtypes
X1     Int64
X2    string
X3    object
dtype: object
```

#### Problem description

NA literal for boolean is not parsed by `pd.read_csv`

#### Expected Output

```python
df3 = pd.read_csv(io.StringIO(txt), dtype={'X3': 'boolean'})
>>> df3
    X1   X2     X3
0  1.0    a   True
1  2.0    b  False
2  NaN  NaN   <NA>
>>> df3.dtypes
X1    float64
X2     object
X3    boolean
dtype: object

# and
df4 = pd.read_csv(io.StringIO(txt), dtype={'X1': 'Int64', 'X2': 'string', 'X3': 'boolean'})
>>> df4
     X1    X2     X3
0     1     a   True
1     2     b  False
2  <NA>  <NA>   <NA>
>>> df4.dtypes
X1     Int64
X2    string
X3   boolean
dtype: object
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.9.184-linuxkit
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : C.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.0rc0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 44.0.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
</details>
