vim: use vim-eunuch
vim: cleanup, comments, fix slowness

* disable folding since I don't use it anyway. using folding=syntax was
  the main reason for some startup slowness for larger files.
* only highlight the first 192 characters of a line (also speed
  related).
* add some plugin comments so I do not forget what each plugin is for.
tmux: decrease pane resize steps, allow mouse for resizing panes
prezto: update to latest commit