Add support for recursive macros

[SergioBenitez]

As in the title. Fixes #129. Note that the changes to meta.rs are not actually necessary - they formed part of an earlier attempt but I decided to keep them here as being able to reference the macro seems like it might be useful in the future. Happy to move those changes.

[mitsuhiko]

Thank you! Sorry I pushed up a few changes to undo the meta.rs so that it gets easier to review. I think the issue here is that at the time of the context lookup the assumption is made that the macro is reachable under that name which however is not necessarily true. Take this for instance:

{% from "helpers.html" import recursive_macro as foo %}
{{ foo() }}}

It would try to look up recursive_macro in the current scope but it cannot be found under that name here. Annoyingly the receiver for call is &self which means we cannot easily put the macro into the context.

I changed it now but there is an awful lot of Arcing going on so I want to check first if that can be avoided.