Disable escaping when rendering view as text/plain

[sorbits]

When calling render on a View or Preview object and giving a text/plain mime-type then I don’t see a need for HTML escaping.

Right now we need to manually disable it, e.g.:

$f3->set('ESCAPE', false);
$text = Preview::instance()->render($template, 'text/plain');
$f3->set('ESCAPE', true);

I request that escaping is enabled only when the mime-type matches the following regular expression: ^(text/html|(application|text)/(.+\+)?xml)$.

If there is agreement I can submit a PR.