Support for addons

[jaroslav-kubicek]

Various users will have various requirements. They should be able to extend our core functionality. I have following proposal:

• user will install addons via npm
• installed addons will be specified in config.cson like this:

testStack:
  addons:
    customReport: true
    targetProcess:
      option: 'value'
      anotherOption: 1

• addons will NOT be invoked by EventEmitter, but specified npm modules will be loaded and functions will be called on specified places - that will enable possibility to abort some predefied core action or change behavior based on return value
• there will be standardized list of functions, that can be called
• each addon could implement only small portion of them, if function isn't defined, nothing will be called at all

[rdpanek]

This is hard topic for now. But it's good idea for next time