Adds tvdb alias for tvdbid and imdb alias for imdbid.
It also fixes an issue where tmdb alias was being ignored
if it was followed by something like "tmdbidfoo".
The same issue prevented imdb pattern matching from
working, if it was followed by something like "imdbidfoo".
It also allows for detecting the first matching occurence,
whether it was an alias or not.
Finally, it ignores attributes with values consisting of only whitespaces.
Jakub Schmidtke
1294990f4f