Add support for outputting subtitles as SubStationAlpha and SubStationAlphav2 (SSA, ASS) formats

[rlaphoenix]

Is your feature request related to a problem? Please describe.
Currently --sub-format has been implemented for all other non-fragmented formats under the Subtitle.Codecs enum, other than SSA and ASS. This is mainly because pycaption does not have a writer for those formats.

Describe the solution you'd like
One potential solution is to use the pycaption.DFXPWriter() then pass the now TTML (aka DFXP) subtitle to https://github.com/Paco8/ttml2ssa

Describe alternatives you've considered
None.

Additional context
If a service provides TTML subtitles then SSA/ASS may be preferred as MKVToolNix doesn't seem to support muxing TTML subtitles (or the spec?). Therefore using --sub-format vtt is currently the only solution to have an MKV while keeping positioning, styling, and all other fancy properties of TTML subtitles. In some communities (namely anime) ASS/SSA is preferred (for whatever reason). Using --sub-format ASS at the moment just logs an error that it is not yet supported and stops devine, unless the subtitle was already ASS.