Add an initial Google convention (#357)

* Add an initial Google convention

Sources for docstring convention:
[Google Style Guide](http://google.github.io/styleguide/pyguide.html#38-comments-and-docstrings)
and [Napoleon Google Style Guide](https://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_google.html)

Rationale for exclusions of errors:
D203: Multiple examples with class docstrings without blank line.
D204: Same as above.
D213: All multiline summaries start immediately after the triple quotes.
D215: N/A no underlines.
D400: First line can end with "period, question mark, or exclamation
point".
D401: Style guide explicitly says not to use imperative mood and use
descriptive mood.
D404: Clearly allowed WRT to the above. There is also an example using
"This".
D406: Section names end with a colon.
D407: N/A no underlines.
D408: N/A no underlines.
D409: N/A no underlines.

* Add D415: Google style summary end char checker.

This commit adds error code D415 which allows for Google
style end char checking (".", "?", "!").

* Add basic google style section checks

* Add support for D417 missing argument docstrings

pydocstyle now supports checking for missing function arguments
from docstrings.

Author: sambhav

.gitignore

@@ -1,4 +1,5 @@
 *.py[co]
+__pycache__
 
 # Vim
 *.swp
@@ -46,6 +47,7 @@ docs/_*
 # virtualenv
 venv/
 .venvs/
+.venv/
 
 # generated rst
 docs/snippets/error_code_table.rst