Detect use of unsafe handlebars (triple curlybraces)

I haven't had a chance to run the code, just test it.

Not sure if that comment body is sufficient, please let me know if I've stepped outside the bounds of the proper format.

Simply put, if you use `{{{fooBar}}}` anywhere in a `.hbs` or `.js` file, it'll make a comment to draw attention to it. There are rare cases you want this, but if it's a mistake this bot is supposed to catch it for you.

Unfortunately `shell=True` is required for any argument to popen that needs to use quotes. The grep's `"{{{~\\?[A-Za-z.]\\+~\\?}}}"` is the offender here.