required() should be a property on CustomArgument
Move uri param processing into an event handler

Previously this logic was baked into the argument
subclass that mapped to parameters in the JSON model.
However, non model arugments should be able to leverage this ability
without subclassing the JSON model specific argument class.

The change here is to move that processing into an event.
There's a `load-cli-arg` event that fires.  The `get_paramfile`
hooks into this and performs its logic.
Use crossplatform tempfile

This will work on windows as well.
Incorporate review feedback
Merge branch 'argproc-enhancements' into develop

* argproc-enhancements:
  Incorporate review feedback
  Use crossplatform tempfile
  Move uri param processing into an event handler
  required() should be a property on CustomArgument