Add join_project_data test; filter "status: Hold"

The "status: Hold" filter is per Leah Bannon's comments in 18F/data-public#5.
Add join_project_data test; filter "status: Hold"

The "status: Hold" filter is per Leah Bannon's comments in 18F/data-public#5.
Merge branch 'hold-projects' of github.com:18F/hub into hold-projects

Conflicts:
	_plugins/joiner.rb
	_test/joiner_test.rb
Merge pull request #17 from 18F/hold-projects

Filter "status: Hold" projects