ScalafmtSbtReporter: mention scalafmt in messages

[kitbellew]

Fixes #138.

[poslegm]

I think that more clear way is creating own logger

class SbtScalafmtLogger {
  def info(message: String): Unit = log.info(s"scalafmt: $message")
  // ....
}

but for such a small amount of code this may be overkill

[kitbellew]

I think that more clear way is creating own logger

class SbtScalafmtLogger {
  def info(message: String): Unit = log.info(s"scalafmt: $message")
  // ....
}

but for such a small amount of code this may be overkill

actually, it's a good idea! let me change that.