Clean-up and add base16 script loading

This removes some accumulated junk and adds base16's 256 colors patch.
Replace vim-golang with vim-go

For some reason the former stopped working for me.
Fix tmux + vim integration and update vimrc

This updates our colorscheme to base16, fixes tmux rendering (I think)
and cleans-up deprecated stuff. I also tweaked some mappings which were
causing issues, added vim-slime etc.