test(coverage): Include human-readable coverage report

While the XML coverage report is useful for consumption by other tools, such as
currently by codecov.io in CI, it's not very useful for humans reviewing the immediate
impact of changes on coverage during local development or while monitoring CI output.  I
don't think running the text report takes much more time so I don't see a downside
here.

.coveragerc

@@ -10,3 +10,7 @@ omit =
      */allmydata/_version.py
 parallel = True
 branch = True
+
+[report]
+show_missing = True
+skip_covered = True

tox.ini

@@ -67,6 +67,7 @@ commands =
          coverage: coverage run -m twisted.trial {env:TAHOE_LAFS_TRIAL_ARGS:--rterrors --reporter=timing} {posargs:{env:TEST_SUITE}}
          coverage: coverage combine
          coverage: coverage xml
+         coverage: coverage report
 
 
 [testenv:integration]