implement a timeout option for `read` ala bash's `read -t timeout`

It would be [useful](http://thread.gmane.org/gmane.comp.shells.fish.user/4022) to have a mode for `read` to fail immediately when there is nothing to read from stdin.

As the simpler version of `read -n 0 -t 0` (from Bash) there could be just a switch (-n? from no wait) to just return !=0 when there was nothing awaiting on stdin (instead of showing a prompt).