Avoid monkeypatching and unify template rendering

[ThiefMaster]

Once a Jinja version containing the changes from pallets/jinja#404 (included in release 2.8) and a Flask version containing the changes from pallets/flask#1422 have been released, the monkeypatches can be replaced with a custom environment.

Additionally, render_plugin_template can go away and be integrated into the custom environment's get_or_select_template method. This would go along with a backwards-incompatible API change in applications using this extension, where render_plugin_template('foo') would be replaced with render_template(':foo') (with the leading : indicating to use the current plugin from the plugin context).