Ignore hidden files and directories based on `show_hidden_files` option.

http://tickets.musicbrainz.org/browse/PICARD-528
Add "Ignore files according to show_hidden_files" to NEWS.txt
Move STYLESHEET_ERROR to parent class as it may be used in any page
Add ignore_regex option to Advanced options

Filepaths matching this regex will be ignored.
Add "ignore_regex ..." to NEWS.txt
Merge pull request #187 from zas/picard528

PICARD-528: ignore file paths if hidden or excluded by regex