Add environment variable 'PLAY_OPTS' to be able to configure system properties in Play.

With PLAY_OPTS, we can override JVM parameters, SBT configuration ... without any modification in Play's scripts.

See also https://github.com/playframework/Play20/pull/1184
Wrap PLAY_OPTS in %ENV_VAR%
Merge pull request #1258 from wsargent/JVM-environment-variables

Add environment variable 'PLAY_OPTS' to 'play' and 'play.bat' scripts