Support for 'section functions'

The mustache manual (http://mustache.github.com/mustache.5.html) describes a feature in the Lambdas section that I think is not implemented in Mu. I'm calling it 'section functions' now, I don't know if that's correct. If I understand the manual correctly, it means that `{{#a}}foobar{{/a}}` with `{ a: function (body) { return body.substr(3); } }` should evaluate to `bar`. I added a test for this behaviour and implemented it. Please let me know what you think.