Added --idea-exclude-maven-target to get exclude IntelliJ from indexing 'target' directories. If you are running maven and pants side by side, these 'target' files contain duplicates of a lot resources and other cruft you don't want to see when you search.

Testing Done:
Manual testing in the square/java repo.

Bugs closed: 570

Reviewed at https://rbcommons.com/s/twitter/r/1017/