Support triple quotes in a template

The following template will produce invalid Scala code:

    """

Since twirl uses triple quotes to delineate raw strings from the template, this will escape out of it.  I think `String.replace("\"\"\"", "\\\"\\\"\\\"")` should be enough to fix it.
Note that this only really impacts the template compiler (unless you actually have triple quotes in your code - but even then I'm pretty sure the Scala compiler would optimise the concatenation of string literals...).