Support --doctest-version in github and bash.

Closes #508.

I included golden tests for `--doctest` both with and without
`--doctest-version`. The without tests will need updating every time
the default doctest version changes. If that's not desirable I can
remove those.

Author: ChickenProp

fixtures/doctest-version.args

@@ -0,0 +1,2 @@
+--doctest
+--doctest-version=^>=0.20