Plugins should be able to add templates to the theme

[ralsina]

These templates should be lower priority than the themes, so themes can actually override them.

My use case is the series plugin: I need a series.tmpl that creates the page for each series, and a series_index.tmpl that has the list of all series.

So, the plugin would provide fallback templates in a templates folder, for Mako and Jinja2, and then the user and the theme maker can override them.

I'll try to make it very painless by just using whatever is in that folder without configuration or extra code in the plugin.