Allow merged DFXP files to use custom TTML

We need this to allow us to use custom styles/regions for our merged
DFXP files.

Refactored the merge_subtitles() method a bit.

Cleanup for the merged DFXP test.  Removed unused variable.  Use the
xmlconst module to get the namspaced attribute names.
Added DFXP merge function to the SubtitleLoader

Refactored SubtitleLoader.load/loads a bit.