Properly trigger pytest matplotlib image comparison (#352)

Adding the `--mpl` flag and saving the failed results under $TESTDIR/results as per https://github.com/matplotlib/pytest-mpl/blob/954073d458ab4d96796c67d6aeb994d17ee39817/README.rst#using.

Update baseline images for grdcontour (5), grdimage (3), logo (2), coast (4), image (1), makecpt (9), basemap(4) and plot(8). Corrected one of the basemap tests that had plotted 'Depth' instead of 'Crustal Age' on the power x-axis label.

Author: weiji14

Makefile

@@ -1,7 +1,7 @@
 # Build, package, test, and clean
 PROJECT=pygmt
 TESTDIR=tmp-test-dir-with-unique-name
-PYTEST_ARGS=--cov-config=../.coveragerc --cov-report=term-missing --cov=$(PROJECT) --doctest-modules -v --pyargs
+PYTEST_ARGS=--cov-config=../.coveragerc --cov-report=term-missing --cov=$(PROJECT) --doctest-modules -v --mpl --mpl-results-path=results --pyargs
 BLACK_FILES=$(PROJECT) setup.py doc/conf.py examples
 FLAKE8_FILES=$(PROJECT) setup.py
 LINT_FILES=$(PROJECT) setup.py