Handle multiline strings with '# noqa'

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