Add flag to dump project info output to file.

This file option will be used in intellij plugin.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/34542731

This is OS Pants review for https://github.com/pantsbuild/intellij-pants-plugin/issues/5

Bugs closed: https://github.com/pantsbuild/intellij-pants-plugin/issues/5

Reviewed at https://rbcommons.com/s/twitter/r/998/