[ALOY-1151] Alloy: Support async build hooks

I love using [Alloy build hooks](http://docs.appcelerator.com/titanium/latest/#!/guide/Build_Configuration_File_(alloy.jmk)) through alloy.jmk files. However, I'd like to also start async processes and block the compile process from continuing until they are done.

Example

task("pre:load",function(event,logger,done){
    require('ticons').assets({
      minDpi: 320
    }, done);
});

Solution

The solution could be modeled after the Titanium CLI which already [supports both sync and async hooks](http://docs.appcelerator.com/titanium/latest/#!/guide/Titanium_CLI_Plugins-section-37549163_TitaniumCLIPlugins-ListeningtoHooks).