Allow a base url for external links

This commit adds the ability to specify a base url for a given data.json to be
used when constructing urls for external links. For example:

  external: {
    data: [
      {
        base: 'http://emberjs.com/api/',
        json: 'http://builds.emberjs.com/tags/v1.5.1/ember-docs.json'
      },
      {
	    base: 'http://emberjs.com/api/',
	    json: 'http://builds.emberjs.com/tags/v1.0.0-beta.6/ember-data-docs.json'
      }
    ]
  }

This is not breaking. For example, the following is still valid:

  external: {
    data: [
      'http://builds.emberjs.com/tags/v1.5.1/ember-docs.json'
    ]
  }
Merge remote-tracking branch 'mmpestorich/yuidoc-237'