Fewer output messages

[lxohi]

scalastyle-sbt-plugin currently prints message in multiple lines, like this:

[info] scalastyle Processed 9 file(s)
[info] scalastyle Found 0 errors
[info] scalastyle Found 0 warnings
[info] scalastyle Found 0 infos
[info] scalastyle Finished in 0 ms

These lines will appears again and again when there were several sub-projects in this build, sometimes even more than one screen.

Do you consider fold these lines into one line is a good idea?

For example:

[info] scalastyle Processed 1 file(s). Found 2 error(s) / 3 warning(s) / 4 info(s). Finished in 5ms

If so, would you mind me create a PR for this change?