{{gh-path}} Handlebars helper, returning paths

closes #3718
- Uses utils/ghost-paths to resolve paths
- Usage: Assume 'http://www.myghostblog.org/myblog/'
- {{gh-path}} or {{gh-path ‘blog’}} for Ghost’s root (/myblog/)
- {{gh-path ‘admin’}} for Ghost’s admin root (/myblog/ghost/)
- {{gh-path ‘api’}} for Ghost’s api root (/myblog/ghost/api/v0.1/)
- {{gh-path 'admin' '/assets/hi.png'}} for resolved url (/myblog/ghost/assets/hi.png)
Yep, I would definitely prefer an unbound helper. Trouble is, we currently don't have any scaffolding for unbound helpers - just making it an unbound helper in utils clashes with other internals that look at all utils and try to register them as bound helpers.

Question: Do you think rewiring that stuff to also accommodate unbound helpers would be worth it? I would like to go down that route, actually.