Linking name in send_message

[Succo]

The send_message method from _channel.py https://github.com/slackhq/python-slackclient/blob/master/slackclient/_channel.py#L23 doesn't use the
link_names argument, which is fine if you can reference user from the api but deosn't work if you only have their name, maybe either making it an option for it would be great or making it the default.

[jaraco]

Here's the send_message method. I'm not sure what you're referring to with link_names. I don't see it in the RTM API docs.