Wrapper to launch an Arcanist Docker container

Summary:
This command allows to use more naturally our nasqueron/arcanist
container: arc shell to enter in a container with bash, or
arc <command> to run a unique command like with the normal arc.

Test Plan: Already tested on Dwellers for some months

Reviewers: Sandlayth

Differential Revision: http://devcentral.nasqueron.org/D350

Signed-Off: Yassine Hadj Messaoud <modepadu95@riseup.net>