Change tabs with modifier+digit

I have a very strong muscle memory for changing tabs via `⌘+1`, `⌘+2`,
etc.

Even though it’s a little strange in the FIFO tabs model I got used to
it pretty quickly.

Here are two of my favorite patterns:

* When tabs are filtered, `⌘+1` always goes to the first match.
* `⌘+2` is always the next tab and allows for easier switching between
only two tabs (just like `⌘+tab`
Merge pull request #19 from nb/modifier-digit

Change tabs with modifier+digit