Missing mechanism to extend the --dist choices when adding a new scheduler results in pytest execution failure #970

Documentation says to use pytest_xdist_make_scheduler to add a new scheduler. However, command line parsing fails because the choices for the --dist command line option are fixed by xdist.plugin.pytest_addoption. Also, help string that documents each of the available options in --dist is not extensible to include the new scheduler choices.

$ pytest --basetemp ~/xdist-debug/basetemp/ -p my_xdist_schedulers.plugin -n 3 --dist mycoolnewscheduler
ERROR: usage: pytest [options] [file_or_dir] [file_or_dir] [...]
pytest: error: argument --dist: invalid choice: 'mycoolnewscheduler' (choose from 'each', 'load', 'loadscope', 'loadfile', 'loadgroup', 'worksteal', 'no')