Darglint

Added

- The ability to read from stdin, which should make it easier to
integrate darglint into other tools.

Removed

- The restriction that a file must end in `.py`.

Changed

- Simplified the interface for the Docstring base class. This should
make modifications easier, and should ensure that the same value is
returned consistently between Google and Sphinx styles.

This comes at the cost of a slightly more complicated function signature
for the functions which remain. However, the single parameter which
is passed to the functions (an enumeration named `Sections`), can
(and should) be used everywhere the `NodeType` enumeration is currently
used (outside of the actual parsing step.) This will effectively create
a firewall around the parsing step.

Fixed

- Fix the parser failing on single-word return description.

Added

- Check for excess variable descriptions in Sphinx docstrings. If the
docstring contains `:var <name>:`, and the *name* doesn't exist in
the actual function, an error will be issued.

Fixed

- Parser exception being thrown when there is a whitespace-only line
with indentation at the same level as an item within a compound section.
(See Issue 7.)

Added

- Support for using Darglint as a Flake8 extension. If Flake8 and Darglint
are installed in the same environment, Darglint will issue warnings through
Flake8.

Added

- Parser for sphinx-style docstrings. This parser handles most
sphinx-style docstrings, but adds certain restrictions. For example,
the fields such as `:returns:` must be the last items in the docstring,
must be together, and any multiple lines must be indented with four spaces.
- Pipfile for setup with pipenv.

Added

- Previously, *darglint* always exited with status 0, even if errors
were encountered. Now, darglint exists with status 1 if docstring
errors were encountered.
- Add description of errors to both the readme and the driver.
The errors are each described on a single line to make it easy
to search for errors from the command line.

Fixed

- Fix regression in type hints: Deque was only available in Python3.6.