Support files lists or tuples in Jinja2 assets block.

E.g.: `{% assets 'f1', ('f2', 'f3'), ['f4'] %}`
renders like `{% assets 'f1', 'f2', 'f3', 'f4' %}`.

It helps assets composition, enabling something like *varargs
(not supported by Jinja2).
Merge pull request #282 from iurisilvio/jinja2_assets_list

Support files lists or tuples in Jinja2 assets block.