Register mpl_image_compare marker to remove PytestUnknownMarkWarning

[weiji14]

Description of proposed changes

Supresses the annoying warning that @pytest.mark.mpl_image_compare emits when running make test because it isn't registered properly.

Original warning was:

/home/.../envs/pygmt/lib/python3.7/site-packages/_pytest/mark/structures.py:324
  /home/.../envs/pygmt/lib/python3.7/site-packages/_pytest/mark/structures.py:324: PytestUnknownMarkWarning: Unknown pytest.mark.mpl_image_compare - is this a typo?  You can register custom marks to avoid this warning - for details, see https://docs.pytest.org/en/latest/mark.html
    PytestUnknownMarkWarning,

-- Docs: https://docs.pytest.org/en/latest/warnings.html

This quickfix suppresses that warning until it is resolved upstream at matplotlib/pytest-mpl#69 by merging in matplotlib/pytest-mpl#83. See also https://docs.pytest.org/en/latest/mark.html#raising-errors-on-unknown-marks

Patches #78

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[seisman]

Looks good to me.