Adding a list of dirs to exclude from the '::' scan.  The CmdLineSpecParser

By default passes along the workdir as the one dir to exclude.
You can add more directories relative to the build root using 'spec_excludes' in pants.ini

Testing Done:
ci is green.

I also created a bogus BUILD file under .pants.d and ran build-support/bin/ci.sh locally and the tests still passed.

Bugs closed: 618, 621

Reviewed at https://rbcommons.com/s/twitter/r/1091/
Sync of https://github.com/pantsbuild/pants/commit/c0762f9f34bc26d17214104d658df064a76a5871