* The easytags plug-in caches known tagged files so it doesn't have to
   run :UpdateTags whenever you edit an existing file. The previous
   implementation was based on the assumption of one global tags file so
   wasn't compatible with the concept of file type specific tags files.
   This should now be fixed.

 * Previously the plug-in worked with a combination of parsed and
   unparsed tags file entries which made the code confusing. I've now
   cleaned this up so that the plug-in only keeps one type of data in
   memory.

 * Moved resetting of s:cached_filenames from the end to the start of the
   functions that call s:canonicalize() to avoid caching invalid data.

PS. I've benchmarked two cache_tagged_files() implementations, one using
    taglist('.'), the other calling xolox#easytags#read_tagsfile() on each
    tags file reported by the tagfiles() function. It turns out that
    taglist('.') is very slow, which explains why I went with the code
    that calls xolox#easytags#read_tagsfile() in a loop.