Index() constructor is inconsistent in detecting subclass-specific keyword arguments
#### Code Sample, a copy-pastable example if possible

```python
In [2]: pd.Index(['20000101', '20000102'])
Out[2]: Index(['20000101', '20000102'], dtype='object')

In [3]: pd.Index(['20000101', '20000102'], freq='D')
Out[3]: Index(['20000101', '20000102'], dtype='object')

In [4]: pd.Index(['20000101', '20000102'], tz='utc')
Out[4]: DatetimeIndex(['2000-01-01 00:00:00+00:00', '2000-01-02 00:00:00+00:00'], dtype='datetime64[ns, UTC]', freq=None)
```

#### Problem description

I see no obvious reason why ``tz`` should trigger the interpretation of data as datetime, and ``freq`` shouldn't. And I think the only viable option is to refrain entirely from using kwargs to do inference - unless we are committed to support them all, which could be a pain.

From the point of view of users, the only change would be to replace
```
pd.Index(['20000101', '20000102'], tz='utc')
```
with
```
pd.Index(['20000101', '20000102'], tz='utc', dtype='datetime64')
```

#### Expected Output

``Out[3]`` and ``Out[4]`` should be instances of the same class.

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

<details>

INSTALLED VERSIONS
------------------
commit: eb0ac5437f9fee2939b3c3c4dab3ed745b5a9c38
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-6-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.24.0.dev0+460.geb0ac5437
pytest: 3.5.0
pip: 9.0.1
setuptools: 39.2.0
Cython: 0.28.4
numpy: 1.14.3
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.2.2.post1634.dev0+ge8120cf6d
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1
gcsfs: None


</details>
