Hack to prevent option defaults from being stomped

If an option is explicitly typed, but no default is provided, then it's
possible for it to return an invalid or misbehainvf value in some contexts
(e.g., returning the string "None" instead of the NoneType instance)

Long term, the option values should be stored inside the bound
descriptors, that will know whether the default value was overridden,
but for now, simply special case `None`
NamedTemporaryDirectory.makedirs()

API for safely making named subdirectories inside a tempdir.
Allow specifying a custom env to twisted commands
Add branch analysis to .coveragerc