Compile then check style task

[steinybot]

I have added a new compileThenCheckStyle task which does what it sounds like it does. There is also a scalastyleCompileArgs which is the input into the scalastyle task. Neither of these names are particularly great, if you come up with a better name I can change it.

I didn't end up using Def.sequential as it only returns the result of the last task which would be Unit instead of Analysis. The idea is the same though, use Def.taskDyn (which unfortunately means that inspect doesn't show it as a dependency).

There are a few tests, some of which were added in a failing state so you can go back to those commits and make sure the tests are working correctly.

I'll submit another PR for the website later.

Closes scalastyle/scalastyle-sbt-plugin#68

[steinybot]

Going through and cleaning up all my old forks. Found this one covered in cobwebs. CI doesn't seem to be working...