contrib: add dokku client shell script

This is adapted from the one I use in my shell profile scripts. This
kinda-sorta-maybe acts like the `heroku` or `deis` commands and does
some clever reorganization of data before sending it to the server.

To use the app name generator, install the `elfs` rock from Moonrocks.
Merge pull request #659 from Xe/client

contrib: add dokku client shell script