Ability for plugins to bundle templates

[simonw]

Plugins should be able to bundle templates.

The Datasette template loader should then consult those plugins first when loading a template.

Jinja2 has a PackageLoader class that can help with this: http://jinja.pocoo.org/docs/2.10/api/#jinja2.PackageLoader

Refs #14

[simonw]

No need to use PackageLoader after all, we can use the same mechanism we used for the static path:

datasette/datasette/utils.py

Lines 694 to 695 in b55809a

	if pkg_resources.resource_isdir(plugin.__name__, 'static'):
	static_path = pkg_resources.resource_filename(plugin.__name__, 'static')