Add `get_text` action to public cursorless api

[pokey]

Could use this api to implement #101

[AndreasArvidsson]

Well you can already call this function from the public api. The problem is that the return value isn't working for any command.

[pokey]

We could try to fold them through the same API, but that would be a change from how we do things internally. Currently only the getText action returns anything even internally. I think for most commands it would be ok to fold them through the same API and just return None, but we wouldn't want to do this for commands that don't wait, and would need to make sure there are no other implications in the command server to always requesting a return value