Add a command to toggle highlight state for currently selected word

[MatmaRex]

Let's add a command that would add the currently selected word to HighlightWords' list (or remove it from it if it's akready there).

I could try coding it up myself if you accept pull requests, but I have almost no experience in Python, and even less writing Sublime plugins, so it might be better if someone competent did that. :)

[seanliang]

This idea was implemented in 2adb3b5