use subtitle settings to set ASS/SSA subtitles position

Many users want to show ass/ssa subtitles on the black screen at the bottom of the video. 
So add a option to set the location of ASS/SSA subtitles on the screen:
In the video: ASS/SSA subtitles will show the position fit the video
In the screen: ASS/SSA subtitles will show the position fit the screen (use the bottom black bar)

So add a function to set the location of ASS/SSA subtitles on the screen use setting 'subtitles.align'.

SUBTITLE_ALIGN_MANUAL
SUBTITLE_ALIGN_BOTTOM_INSIDE
SUBTITLE_ALIGN_TOP_INSIDE: 
ASS/SSA subtitles will show the position fit the video

SUBTITLE_ALIGN_BOTTOM_OUTSIDE
SUBTITLE_ALIGN_TOP_OUTSIDE: 
ASS/SSA subtitles will show the position fit the screen (use the bottom black bar)

can we align the `subalign` please