Bring more up-to-date with the muddle equivalents

1. A couple of minor docstring fixes (very minor)
2. Add the "keep_anyway" option for TransientDirectory - this allows
   (for instance) passing a "-keep" switch to an application, and
   passing its value to this argument, neatly stopping the directory
   from being transient. Which is occasionally useful.
3. Add the new counted directory class.