Allow configurable repository links

The default link is to `/` in the header but if hosting on gh-pages, that will link to your `.github.io` which might not have anything to do with this package. Example: http://mhkeller.github.io/indian-ocean/docs/

It would better if it could add the repository link somehow — either through the same method as [streams/github.js](https://github.com/documentationjs/documentation/blob/master/streams/github.js), parsing the `package.json` or through a command-line flag.

If this url were exposed in the handlebars `index.hbs` file, another option would be to let theme-builders take implement this and not built it in as a default option. I tried to use `context.github` in that file but that seems to be more narrowly scoped.