add python.app as an output

https://github.com/conda-forge/orange3-feedstock/pull/54 ran into some difficult-to-find errors which ultimately boiled down to `python.app` coming from the defaults channel while `python` came from conda-forge/label/gcc7. If that happens, pythonw doesn't work at all (the immediate failure is due to strict regex parsing in the patched platform.py), so it would probably be good to try to express python.app's extremely strict dependency on a specific build of python.

Now that we have multiple build outputs from conda-build 3, it probably makes sense to have `python.app` as an output of this recipe with `pin_subpackage('python', exact=True)` rather than an independent feedstock that is allowed to install with different Python.