Add support for explicit inclusion

This small patch adds support for explicit _inclusion_ of Jinja2 templates.  This is useful in situations where you are migrating to Jinja2 gradually.  Instead of excluding apps, you can use this to partially migrate an app (i.e. some views use Jinja2, the rest still uses Django).

When `JINGO_INCLUDE_PATTERN` is set, Jingo is only used for files that match that pattern.