directedMessage accept array of target strings

[cfindlayisme]

From #17

Ie, currently the struct is

type DirectedIncomingMessage struct {
	Target          string
	IncomingMessage IncomingMessage
}

Which currently looks like this for the payload you'd send:

{
  "Target": "#exampleTarget",
  "IncomingMessage": {
    "Message": "Hello, world!",
    "Password": "examplePassword",
    "ColourCode": 3
  }
}

I'd also like to accommodate Target being an array vs just a single string, so that we can in one JSON payload send a message to multiple targets (ie channels, users.) So it would look like this:

{
  "Target": ["#exampleTarget1", "#exampleTarget2"],
  "IncomingMessage": {
    "Message": "Hello, world!",
    "Password": "examplePassword",
    "ColourCode": 3
  }
}