Evaluate pylint versus flake8 for common uses

[brettcannon]

E.g. which is faster while covering the common linting rules that the vast majority of people care about.

[MikhailArkhipov]

Pylint appears to be significantly more popular. Performance-wise flake8 seems to be faster but there is not much difference on small files and projects and on large projects one would probably use pylint anyway since it provides deeper analysis.