Add option to keep pass statements

[alvarotroya]

Changes in this PR:

• Add two flags to customize autoflake's behavior for pass statements:
  - ignore-pass-statements: allows the user to opt out of the removal of useless pass statements if desired.
  - ignore-pass-after-docstring: allows the user to keep pass statements after a docstring (see below for caveat).
• Add tests for new flags.
• Update README.

Caveat: since autoflake does not use an AST, deciding whether a pass statement follows a docstring is not really possible (at least not with a LOT of effort), so the approach is to keep pass statements right after a line ending with triple quotes. This is most probably in 99% of the cases a pass after a docstring.

This adds the possibility of keeping pass statements after docstrings which are not useless and is one of the requested features in the repo, see #73, #10. I know that autoflake will already keep the pass statement if:

• I leave an empty line after the docstring, but this goes against PEP 0257,
• or if I use a docstring format which uses # instead of triple-quotes, which is not the case for anyone using Google Style Docstring

Closes #9.
Closes #10.
Closes #73.

[fsouza]

Thank you!

[alvarotroya]

Thank you!

my pleasure!