initial import of tags-from-filename plugin (#113)

This was contributed via email. Now I'll clean it up for inclusion.
fromfilename: python style, naming, comments
fromfilename: switch to declarative style

No need for redundant handleXfields methods; we'll just inspect the fields
we're given.
fromfilename: preserve case, apply in more cases
fromfilename: simplify
fromfilename: docs/changelog (closes #113)