Add interactive execution of commands

[Olivia5k]

This PR adds a few commands to execute things from docker-compose files, via interactive selection. You can select a command to run on all services, or just a specific one. The selection itself is using completing-read, so it should hook into helm, ivy, ido or whatever the system of the future might be.

The commands use a predefined list (gotten from copy-pasting from docker-compose help, hehe) and the services use docker-compose config --services to get a list of the services.

A special string docker-compose-all-services-token, which defaults to "*all services*" is prepended to the services list. When selected, it signals that the command is to be run on all services (e.g. with no further arguments to the command line).

Glorious gif |o/

Please excuse the slight jittery nature of the gif. It's literally the first gif I've ever made, and I hope it gets the point across anyway.

For trivia, I've based the code and functionality on my similar makefile-executor.el project, which does this exact thing but on Makefile targets.

Closes #19 since it contains an improved version of it.

[pataquets]

Check out functionality on Silex/docker.el package.