Replace add/remove_parens with toggle_parens.


![coffee](https://cloud.githubusercontent.com/assets/3876443/4013617/48a3b126-2a18-11e4-827d-c9cb6d4d3e7f.gif)


With this, parens are removed if your cursor selection is empty, and added if you have a selection. **I don't think this is the most intuitive way to toggle.** But I'm not sure there's a better way to determine whether arguments are parenthesized or un-parenthesized. The best solution might be to add a syntax-highlight definition that would match function arguments and differentiate between those in parens and those outside of parens. This is beyond my regex abilities currently.