RocketChat compatibility.

[pawstern95]

Some time ago our team moved from Slack to RocketChat. Since RocketChat has Slack Compatible API we wanted to use tcSlackBuildNotifier to notify about our builds. In general it works but links to our builds are not properly formatted and do not directly lead to build, we get Build with id 'null' not found instead. After some investigating I discovered that your plugin uses Slack formatted links instead of Markdown. I also found out that RocketChat parses & in Slack links to & so this is the reason why links do not work properly.
I was able to find the place where links are generated and change format to Markdown myself. In our teamcity instance it works well. The only issue was I had to change brackets in which branch name was placed from square [] to parantheses () because escaping did not work in my RocketChat instance.