Extract Playframework plugin to a separate github module

Hi!

I suggest to extract "Playframework support" plugin to a separate module.

As a committer of Play 1 framework, I would like to also update the IDEA support. Currently this plugin is built-in IDEA, meaning that I cannot change its sources or send pull requests (am I right?).

At the same time, I don't think JetBrains really wants to spend its time for maintaining Play1 support (as it's not really popular framework nowdays). So, it's win-win deal for JetBrains.

https://www.jetbrains.com/help/idea/2016.3/play-framework-1-x.html