Implement action func parameter conversion
Implement action func parameter conversion

Implement the 'convert' argument of the action function decorator. This
argument has always been present, but it's logic wasn't implemented.
This integrates a call to the conversion function in
_map_params_to_view_args.

Should the conversion raise an exception it is stored in a new
_convert_errors context local and thus available for graceful handling
by the caller. This follows the pattern in flask-ask (http://flask-ask.readthedocs.io/en/latest/requests.html#mapping-alexa-requests-to-view-functions).

treethought/flask-assistant