Themes

Added theme support. I had reservations about doing this since it makes things a little more complicated with the introduction of a config file (docs/.viewdocs [or maybe it should be at root so we could add another config like `docsPath: documentation/`]). But I was thinking about the non-creative guys that find it hard to design their own templates but want something different looking.

Some notes:

* Config is from through `docs/.viewdocs`
* Config is a yaml file
* Theme templates are downloaded on app start like how the default template was (how about using https://github.com/jteeuwen/go-bindata ?)
* Added theme `yue` from https://github.com/lepture/yue.css
* Example of repo with theme set as `yue`: https://github.com/marksteve/viewdocs-test