Implement a basic document statistics tabulator. (Fixes #211)

As requested in issue #211, I've implemented a basic document statistics feature in the sandbox. It does line count, word count, and character count with/without spaces (so in total it gives you 4 different numbers about your document). This could probably be expanded with other metadata-y information, but this was all I could think off the top of my head, and it satisfied the basic request for a word counter. 

I gave it a keybinding of F2, that was mostly so I could test it more rapidly, not married to that key though, let me know if you want me to change it.