async plugin: adding second parameter to support naming a global callback function instead of the generated one.
Merge branch 'master' of https://github.com/mhrisse/requirejs-plugins

# By Matthias H. Risse
# Via Matthias H. Risse
* 'master' of https://github.com/mhrisse/requirejs-plugins:
  async plugin: adding second parameter to support naming a global callback function instead of the generated one.