Add a `mc` command that replicates `mix cmd --app my_app`

[nicholasjhenry]

command line: mc <app_name> <command>

e.g. deamc my_app mix test.watch --only wip

• changes directory to apps/my_app
• runs the provided mix command
• returns the original directory
• also provide a deamc version for Docker

[frahugo]

Handled in #17