Remove old tmux config files
A bunch of commands I defined are already in eunuch.vim
Add the SCSS syntax for vim

Before this commit, SCSS (and Sass) syntaxes were handled by Tim Pope's vim-haml
plugin, which wasn't good with SCSS highlighting. Since I nearly never use Haml,
for now it's better to use a dedicated SCSS plugin which highlights stuff
correctly, like `%placeholder-classes`.

Closes #17.