sub: sub-filter-regex and jsre: support ass-to-plaintext

Using --sub-filter-regex-plain (default:no)

The ass-to-plaintext functionality already existed at sd_ass.c, but
it's internal and uses a private buffer type, so a trivial utility
wrapper was added with standard char*/bstr interface.

The plaintext can be multi-line, and the multi-line regexp flag is now
always set, but only affects plaintext (the ASS source is one line).