Add dokku run command to run commands in the app env

Depends on https://github.com/progrium/buildstep/pull/27 being merged and https://github.com/progrium/pluginhook/issues/3 to be fixed.

Allows you to run cli commands in the environment of the app as:
`dokku run <app-name> <cmd>`

Example:
`dokku run-cli php-hello-world printenv`

~~Currently named `run-cli` to be consistent with the naming in buildstep. Open for other alternatives (cannot be `/run` as that's a directory).~~

Edit: run-cli -> run now