Universal Text Subtitle Support Initial Implementation

Universal Text Subtitle Support Initial Implementation
- Patch by davidfstr ... Thanks!
- Adds support for reading TEXT subtitle tracks from file inputs
- Tested combinations:
-- MKV UTF-8 -> MKV UTF-8 (passthru)
-- MKV UTF-8 -> MP4 TX3G (upconvert)
-- MP4 TX3G -> MKV UTF-8 (downconvert)
-- MP4 TX3G -> MP4 TX3G (downconvert to UTF-8 then upconvert)
- Further explained here http://forum.handbrake.fr/viewtopic.php?f=4&t=16099

git-svn-id: svn://localhost/HandBrake/trunk@3283 b64f7644-9d1e-0410-96f1-a4d463321fa5