Plugin config should support promises.

[JoshMcCullough]

Current behavior:
Plugin configuration callbacks are expected run synchronously.

Expected/desired behavior:

• What is the expected behavior?
  The callback passed into aurelia.use.plugin to configure the plugin should allow for the return of a Promise. Configuration of Aurelia should not be considered "done" until all plugin configuration callbacks have been resolved or rejected.
• What is the motivation / use case for changing the behavior?
  To allow for more complex/async configuration of plugins, before Aurelia is loaded.

Example

aurelia.use
    .standardConfiguration()
    .plugin('aurelia-configuration', config => {
        config.setDirectory(System.baseURL + '/config');
        config.setConfig('application.json');
        config.setEnvironment('development');

        // Returns a promise.
        return config.mergeConfigFile(`${config.directory}/application-overrides.json`);
    });

return aurelia.start().then(a => a.setRoot());

Workaround

let promises = [];

aurelia.use
    .standardConfiguration()
    .plugin('aurelia-configuration', (config) => {
        config.setDirectory(System.baseURL + '/config');
        config.setConfig('application.json');
        config.setEnvironment('development');

        promises.push(config.mergeConfigFile(`${config.directory}/application-overrides.json`));
    });

// Allow any async configuration to complete before starting Aurelia.
return Promise
    .all(promises)
    .then(() => {
        return aurelia.start().then(a => a.setRoot());
    });