Add rootRelative option to specify where helpers/partials resolved from

Currently, the loader will always try to automatically resolve helpers/partials relative to the template file's directory when it encounters a tag like `{{title}}`. However, sometimes you may not want it to pull in `./title.js` automatically, because it might not actually be a handlebars helper.

`rootRelative` can now be specified to override this default lookup behavior. Specifying `rootRelative=` on the loader `query` will effectively turn the behavior off entirely, or it can be set to any path you choose.

By default it is set to `./`, maintaining current compatibility.