Add 'how' option to 'first_valid_index'
This is a suggestion to include a `how` option to [`first_valid_index`](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.first_valid_index.html#pandas-dataframe-first-valid-index) ([similar to `dropna`](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.dropna.html#pandas-dataframe-dropna)), such that the first index having at least one non-`nan` value (`how="any"`, current behaviour), or no `nan` value (`how="all", new behavious), is returned.

#### Code Sample, a copy-pastable example if possible

Currently, this can be accomplished as follows:

```python
df.dropna().first_valid_index()  # would become df.first_valid_index(how="all")
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.8.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-54-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: pt_BR.UTF-8
LOCALE: pt_BR.UTF-8

pandas: 0.22.0
pytest: 3.3.2
pip: 9.0.1
setuptools: 39.0.1
Cython: None
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 5.5.0
sphinx: 1.6.7
patsy: 0.4.1+dev
dateutil: 2.6.1
pytz: 2018.3
blosc: 1.4.4
bottleneck: None
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.1
openpyxl: 2.4.9
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: 4.2.1
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
