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.
I think the performance of this code should be OK, but it would be good to have performance regression tests for Twirl templates.