Update emacs configuration

Putting things in version control is a good idea when you're not
familiar with the editor, to prevent *data loss* :P
Emacs related TMUX tweaks
Better line numbers and current line highlighting

This still doesn't look nearly as good as my VIM setup, but I can live
with it.
Improve our `H` and `L` overrides

Sometimes I'll get confused. It's nice to have more keys mapped to the
same actions.