Add support for setext headers to the TOC command.

I took the base of your TOC function for vim-pandoc/vim-pantondoc, adding setext headers support. I thought it would be nice if I shared that back (the full implementation is here: https://github.com/vim-pandoc/vim-pantondoc/blob/master/autoload/pantondoc/toc.vim, you'll see there are some additional differences). Feel free to clean it up.