Report coverage (#863)

Makefile

@@ -4,6 +4,9 @@ test:
 test_nvim:
 	VSPEC_VIM=nvim pytest test_*.py
 
+test_coverage: export PYTEST_ADDOPTS:=--cov pythonx --cov test --cov-report=term-missing:skip-covered
+test_coverage: test_nvim
+
 build:
 	mkdir $@