builddict: add a flag to omit "internal" things

An option for goal builddict, the goal which generates http://pantsbuild.github.io/goals_reference.html . This option is a list of regexps; goals whose implementations (e.g., 'pants.backend.jvm.tasks.benchmark_run.BenchmarkRun') match any of these regexps get omitted from the goals_reference.

Why? To prevent 'sitegen' from showing up on http://pantsbuild.github.io/goals_reference.html . The 'sitegen' goal is defined in internal_backend, which we don't ship with Pants.

Bugs closed: 777

Reviewed at https://rbcommons.com/s/twitter/r/1546/