Allow sending direct message to user

[atomasevic]

Opening new PR after squashing commits and closing PR on master branch.

Original PR text:

I wanted to send a direct message to user, and if I specify channel as @slackusername, your method setDefaultOptions was adding # sign to beginning of my channel name, so final channel would look like #@slackusername.

I added one more check if channel starts with @ to avoid adding #, so we can send direct messages to user.

[mcrumm]

Thanks!