Add `ignore-self-only-init` option

[thejcannon]

Adding an option ignore-self-only-init (off by default) which will not report a D107 error if __init__ has no parameters other than self. The justification is that these methods usually aren't very interesting and will likely result in redundant/boring docstrings which add noise.

Just turning off D107 isn't an option, since there are __init__ methods which are interesting ;)

Please make sure to check for the following items:

• [x] Add unit tests and integration tests where applicable.
• [x] Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".

[thejcannon]

Of course, the option name is up for debate :)

[sambhav]

@thejcannon could you please rebase this PR against the master branch? Thanks!

[thejcannon]

Better late than never :sweat_smile:

[thejcannon]

@samj1912 got time for a quick review?