Add pytest.ini support for --reruns and --only-rerun options

[david-vankampen]

Currently the options are only available on the command line (and thus must be provided every time). If support were added to read these settings from a pytest.ini file, the developer wouldn't need to remember to add them every time. It can be done indirectly through the "addopts" arguments, but it would be more clear/self-documenting if they were distinct options as well.