This code adds the ability to add Mako template lookup directories on
the fly, allowing third party add-ons to contribute their own Mako templates.
A new API function for registering Mako templates is introduced::
from edxmako import add_lookup
add_lookup('main', '/path/to/templates')
# Or, specify a package to lookup using pkg_resources. This will
# add the 'templates' directory inside the current package:
add_lookup('main', 'templates', package=__name__)
