In Flake8 2.x we allowed people to use # noqa at the end of a multiline
string to ignore errors inside the string (e.g., E501). Being blissfully
ignorant of this, I never accounted for it in Flake8 3. This fixes the
oversight and allows multiline statements to have the # noqa at the end.

Closes #177

The documentation for the `FileProcessor` class used `indect_char` while the
class itself uses the more sensible name `indent_char`. This updates both the
docstring as well as the documentation.

Yesterday we released the flake8-polyfill package to help with Flake8
compatibility issues. This adds documentation to Flake8 to help people
use that and to guide them towards it.

When calling `add_option` it returns an `Option` object which can return the
primary name of the option via `get_opt_name`. This should be used primarily
in the cross compatible implementation so that the order of parameters does
not matter.

There are now a lot of moving parts in Flake8. It can't help to give new
developers a high-level overview of how they all fit together to make
Flake8. =)