Add support for Haml as templating launguage (using lua-haml)

[daelvn]

(Related to #47) I think that this would be a neater approach for template generation, and it wouldn't be much work, using lua-haml. The implementation would consist of the features/haml.moon file (for .haml files), and the haml.moon widget extension. As far as I'm aware, it wouldn't be required to use the Buffer class, only the renderer that takes options, and the ability to include self.

[leafo]

If you want to contribute a haml feature file I'd be glad to take a look. I'm not using haml personally so I'm not too familiar with any of the existing modules.