Add flake8-rst for docstring code examples

[kataev]

Hello, i'm created new tool flake8-rst.
Idea is simple, crop with regex code snippets and put it in flake8.
PR splitted into logical commits for better review. First commit, add sourcecode:: pycon, next fix found issues.

[menshikh-iv]

Nice work @kataev 👍 definitely useful addition!

Please also add this check to CI here https://github.com/RaRe-Technologies/gensim/blob/develop/.travis.yml#L16

[menshikh-iv]

Notes (mostly for me)

• Choose "good-enough" ignore parameters for config & raise an issue about eliminating part of it (for example, we can't immediately fix all docstrings, some of "undefined name" fix pretty hard). Done
• Move flake8-doc to python2 (same as flake8 works now), for this, I'm waiting for "python2-compatible" release of flake8-rst from you @kataev Done, now we have checks on 27 and 36
• Resolve confict & merge PR with the fresh develop Done

Next great step (after this PR) will be #2107, @kataev have you time to help us with it?

[menshikh-iv]

Great work, thanks @kataev 🌟 Congratz with first contribution 🚀