don't fail on unicode js newlines (\u2028, \u2029)

JSON isn't a proper subset of JavaScript, and JSON.stringify
is horribly broken by default.

So when those two characters were used, Jade was creating
a template with syntax error (with valid JSON, but invalid JS).
Merge pull request #1655 from rlidwka/nl

don't fail on unicode js newlines (\u2028, \u2029)
First class mixins