cheap way to give access to ^W, etc; closes #29, #25

[doriantaylor]

Describe the change

This is a trivial change that affords ^W etc; closing #29 and #25.

Why are we doing this?

Users expect common shell key bindings to be available.

Benefits

The ability to set said key bindings.

Drawbacks

Don't know. There is a tradeoff between passing line into the key event rather than line.to_s and the downstream expectation that nothing is going to change that value. I had to change the unit tests to pass, but at a glance that seems more reasonable. I probably would have designed it so the return value of trigger is captured as a replacement for the line, but introducing that now would probably break more stuff downstream.