elpy: add elpy-buffer-args for easy control of _main_

The following patch enables the user to define elpy-buffer-args so that
when a users runs C-u C-c C-c the sys.argv[] array is first set.
Although this can be done manually this seems like a useful
optimisation.

Open questions:

  * Should I also define a history variable for the arguments?
  * Should there be a default key-binding?