Make fig run COMMAND parameter optional.

This behaves more like the native docker client, where the absence of a
command means docker runs the CMD in the Dockerfile. If a command is
defined in fig.yml this is used instead.

Signed-off-by: Chris Corbyn <chris@w3style.co.uk>
Merge pull request #283 from d11wtq/feature/optional-command

Make fig run COMMAND parameter optional.