Add start, down aliases for up, stop

[glasnt]

Normally CLIs have start/stop or up/down.

In divio-cli's case, it's up/stop, so I propose adding aliases for start and down. Both do the same thing as their counterparts.

[jmelett]

We were trying to be consistent with docker-compose here (e.g. https://docs.docker.com/compose/reference/stop/), happy to discuss it if the team thinks it needs to be changed.

[kinkerl]

docker-compose has up/down and start/stop pairs

https://stackoverflow.com/questions/33715499/what-is-the-difference-between-docker-compose-up-and-docker-compose-start