support html input

[notslang]

For people wanting to use custom compilers or just need html-to-markdown conversion. Thanks to #26, this should be easy. Something like --input=html would make sense (and just have the default be --input=markdown), but maybe a plain flag would be better.

We could also do some autodetection by reading the first chunk of text from the CLI & looking for a doctype... idk how reliable that would be.