Plac

Improved the interactive experience with full readline support and
custom help. Added support for long running command, via threads and
processes

Gigantic release. Introduced smart options, added an Interpreter class
and the command container concept. Made the split plac/plac_core/plac_ext
and added a plac runner, able to run scripts, batch files and doctests.
Removed the default formatter class

Fixed the installation procedure to automatically download argparse
if needed

Added missing .help files, made the tests generative and added a
note about Clap in the documentation

Changed the default formatter class and fixed a bug in the
display of the default arguments. Added more stringent tests.

abbrev is now optional. Added a note about CLIArgs and opterate.
Added keyword arguments recognition. ``plac.call`` now returns the
the output of the main function.