tmux: renumber windows automatically

Most of my window switching is done using `prefix ^a` (ie. the
`last-window` command), so the ability to jump to a specific window
using a number is less valuable than having the numbers grouped close
together (where accuracy is higher).

Signed-off-by: Wincent Colaiuta <win@wincent.com>
tmux: use abbreviated host name on status bar

The host name itself is usually specific enough; out with visual
clutter.

Signed-off-by: Wincent Colaiuta <win@wincent.com>