Change main DownloadBuilder API to accept config as option, and to let config tell a path to be used as the prepared source

Previously, main was `frontend.js`, now it is `main.js`. Frontend can now be accessed with `download.frontend`, where `download = require( "download.jqueryui.com" )()`;

The new API:

require( "download.jqueryui.com" )( options )
- options [ Object ]: key-value pairs detailed below.

options
- host [ String ]: optional, specify the host where download.jqueryui.com server is running. Default: "" (empty string).
- config [ Object ]: optional, if present used instead of the `config.json` file;

On the config, a prepared path can be specified for stable or legacy by using the `path` attribute, instead passing the version plus letting `grunt prepare` prepare it. Eg.

download = require( "download.jqueryui.com" )({
   config: {
       "jqueryUi": {
           "stable": { "path": "my/stable/path" }
       }
   }
});

The above prevents jQuery UI of some workarounds on its grunt `release_themes` task.