Templates and mixins

[forste]

Added support for templates and mixins. See Readme for more.

[tj]

looks decent I'll go through and review!

[forste]

let me know if characters [+~#] or strings (which ones) are prefered. I can fix the rest then and re-request

[forste]

additionally I thought about allowing compile templates and mixins so that they don't have to be composed every request. compilation would be independent from caching mechanisms

[rummik]

@forste From the way things read, strings are the preferred