Customize pyflakes/pep8

This commit adds a `pep8_select` setting, which is the inverse of the `pep8_ignore` setting. I've also renamed the `pyflakes_ignore` setting to `pyflakes_ignore_globals` to be more accurate, and created a new `pyflakes_ignore` setting for ignoring all pyflakes warnings and errors on a single line. (This new setting defaults to the string "# pyflakes.ignore", for compatibility with [jehiah/pyflakes](https://github.com/jehiah/pyflakes).)