Add support for escaping closing braces

Since Twirl uses closing brace to end blocks, if you want to put a closing brace into a block as ordinary text, you need to escape it.  This provides a convenient mechanism for escaping it, using @} as syntax.
Actually, I'm struggling to work out exactly what that expression does... it seems to match any character that is not at, but ignores it, followed by any character that is not { or }, and ignores that, followed by any character.... but that can't be right.