Add content_filename to transmission plugin.

Add content_filename to transmission plugin, mimics the behavior of deluge plugin.
Tested with:
-Transmission 2.84
-TransmissionRPC 0.11
Raise loglevel for message about changing loglevel.
Try something else to troubleshoot ref #2660
Added keep_extension option, default true

Sometimes it's desirable to be able to change a files extension with move plugin
Prevent imdb_list from crashing on movies with no year
Switch imdb_list to html5lib parsing and hack it to work