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.
Suggestion for making this doc-string more readable:

If the selection is empty, removes all text from the cursor to the end of the line. If the cursor is already at the end of the line, it removes the following newline. If the selection isn't empty, only deletes the contents of the selection.