Run doctests for filters, etc

Author: sgillies

tox.ini

@@ -14,4 +14,4 @@ python =
 deps = pytest
 commands =
     python -m pip install -e .[dev]
-    python -m pytest -v -rxXs --cov planet --cov-report term-missing
+    python -m pytest -v -rxXs --doctest-modules --cov planet --cov-report term-missing planet tests