Add tmux `reattach`

[cceckman]

tmux has a useful "session groups" feature, where you can have different views of the same set of windows. This can be useful to have non-square layouts of a single workspace, e.g.:

+---------+--------+
| browser | `ws X` |
+---------+        |   
| `ws X`  |        |
+---------+--------+

It looks like tmux new-session -t 'session-name' is the way to do this: https://unix.stackexchange.com/questions/24274/attach-to-different-windows-in-session

My attach alias uses -D to boot out any other client attached to the session, and I'm good with that; I should add a reattach that doesn't include it.