Add Maven completion

Closes #39.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
New Formula for packer-completion.

This formula provides bash completion for packer (http://packer.io) and
supports all commands and options of packer-0.6.0.

Closes #41.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>