make rebuild_tags() optionally accept 2 lists (of filenames and tags)
tags_in_post() to list all tags referenced in one post file
posts_with_tags() to list all posts referenced by several tag files
use new functions

also bugfix: leave only unique filenames in rebuild_tags()
don't rebuild tags if they were not involved at all

(otherwise posts_with_tags would endlessly freeze)
proper check for empty variable
fix typos in comments
Merge pull request #52 from Lex-2008/rebuild-less-tags

Rebuild less tags