Add a `run` command to the cli

[gmolveau]

It would be useful to run directly a command on a specific app instead of having to log with ssh then run the command, if it's possible :)

[Keirua]

This is (now?) possible through a pipe, maybe it could be documented somewhere:

echo whoami | clever ssh --alias your-app-alias
[…]
bas

bas is the name of the user.