Adds alias support

[jfryman]

This commit adds the ability for Hubot to respond to messages directed at it via an alias. This command used to be supported with the --enabled-slash parameter, and as a result the slash '/' was fittingly chosen as default.

This value can easily be changed via the parameter 'hubot::chat_alias' defined in either hiera or at class declaration.

The parameter name chat_alias was chosen over alias, as the latter is a Puppet metaparameter.

Examples:

---
  hubot::chat_alias: '!'

  class { 'hubot':
    chat_alias => '!',
  }