Use ms-pasteTextOnly to fix text pasting issues on IE11

Supposedly can fix #298. Pinging @anitin and @jonathansampson for help on checking this as well.

Long story short, `insertText` command is not supported (?) in IE11. Went to the documentations on MSDN on execCommand and found `ms-pasteTextOnly`. 

Works like a charm for me, but I haven't thoroughly tested the whole thing to confirm that it doesn't break support on other browsers and so on. Go ahead and review it first.