Add support for Nexusmod's BBCode format

[cloudyluna]

Thanks for this project! :smile:

I have some LaTeX files written in to convert to Markdown files. Those worked nicely. I also tried to compile those tex files to Nexusmod's own BBCode format with all three provided bbcode lua parsers, but none of them produces completely correct markup elements (Nexusmod's doesn't use [h1] for some reason). I had to adjust some things in output manually but it would be nice to have Nexusmod's format supported too! :slightly_smiling_face:

Nexusmod's BBCode format reference