added delete to end of line

This adds a command to delete text to the end of the line, without modifying the clipboard. This is the behavior in Sublime Text and what I was used to.

I'm not sure how to run the tests (`npm test` throws a whole bunch of weird errors unrelated to this commit). Perhaps someone familiar with this can run them, and/or tell me how to, in case I need to fix something? It does have the behavior I expected using it on my own.
I feel like `deleteToEndOfLine` should work in all cases identically to `cutToEndOfLine` (with the obvious exception of not modifying the clipboard). In this case, `cutToEndOfLine` only cuts the *selected* text, not to the end of the line.

See: https://github.com/adnelson/atom/blob/feature/delete-to-end-of-line/spec/editor-spec.coffee#L2143-2151