Added 'exclude-folders' option in pants.ini, defaulted to excluding a few dirs in .pants.d

 This is a followon to https://rbcommons.com/s/twitter/r/1017 that gets rid of the fixed list of includes in the .iml mustache template
 and allows you to override it.

Testing Done:
Updated the integration tests and inspected manually.

Reviewed at https://rbcommons.com/s/twitter/r/1023/