Use colordiff (everywhere)

This used to be re-set in host-specific bashrc, but we already rely on
so many things installed anyway...

dotfiles/bash/main.bashrc

 alias ll='ls -lh'
 alias lla='ls -lha'
 alias open='gnome-open'
-alias diff='diff -u'
+alias diff='colordiff -u'
 alias dmesg='dmesg --time-format iso'
 alias pad4='sed -e "/./s/^/    /"'
 alias grep='grep --color --binary-files=without-match'