Support subscripts

[dwhieb]

Support subscripts, perhaps with word_sub_ notation, but this might be problematic because it conflicts with the use of the underscore in Leipzig glossing conventions.

Also, the dlx2html library currently supports underlining using underscores too.

Markdown's word~sub~ may not work because the symbol for reduplication is ~. However, subscripts require two of those, so it might work if you simply process the subscripts first. Currently using underscores breaks the dlx2html library because it treats those as morpheme boundaries for reduplication.

Maybe you could support this via a preprocessor or postprocessor.