Add function to pluginHelper

[achambers]

This PR will add a readConfig function to the pluginHelper object that is passed to plugin functions.

This function would be used by plugin authors and users that would like to access the config for the current plugin. An example of this is as follows:

host: 'localhost',
port: 1234,
redisDeployClient: function(context, pluginHelper) {
  var redisLib = context._redisLib;
  var options: {
    host: pluginHelper.readConfig('host'),
    port: pluginHelper.readConfig('port'),
  };

  return new Redis(options, redisLib);
},

This is instead of doing the old way:

host: 'localhost',
port: 1234,
redisDeployClient: function(context) {
  var redisLib = context._redisLib;
  var options: {
    host: this.readConfig('host'),
    port: this.readConfig('port'),
  };

  return new Redis(options, redisLib);
},

which no longer works since ember-cli-deploy/ember-cli-deploy-plugin#15 was introduced.