Add a way to expand tokens #751

Let's say the binding is CTRL-X.

Now we can be more selective: rm *.png CTRL-X rm a.png bc.png foo.png CTRL-W rm a.png bc.png

We can confirm that we typed the right thing before executing: mkinitrd (uname -r) CTRL-X mkinitrd 3.8.11-200.fc18.x86_64

Or that we typed enough: kill %fi CTRL-X kill 1580 1684 2021 3431 3822 oops I only wanted firefox

Possible problems to solve:

• Evaluation has side-effects (at least for subshells) such as printing to stderr. Completions have the same problem though.
• Are the problems with eval relevant to this?
• The result needs to be properly quoted or escaped so that it works exactly like running the commandline without expanding anything (other than the obvious stuff like (date) depending on when it's run).

This idea came out of #731 but I now think they're two separate ideas.