Special treatment for multiline strings (issues #224 and #242)

This patch series fixes both issue #224 and #242, mainly by introducing special recognition of multiline strings. There are two intended behaviour changes:

1. a line inside a multiline string that consists entirely of non-whitespace chars, except for leading indentation, will *not* be checked for max length: this is the default and cannot be turned off!
2. putting "# nopep8" (or "# noqa") on the last line of a multiline string disables all physical checks for lines in the string -- but note that the terminating line (with the closing ''') is still checked, on the grounds that its newline is not in the multiline string therefore is not part of it

If you like this approach, I'm happy to supply a doc patch too. Will wait for thumbs up or down before I tackle the docs though.