Add a quiet flag to remove superfluous output

Currently `deis` commands output a "header" as the first line of output, for example `deis apps:list` will output something like:

```
=== Apps
casual-quacking
hidden-mongoose
zephyr-instinct
```

In some situations command may also output some warning:

```
! WARNING: Client and server API versions do not match. Please consider upgrading.
! Client version: 2.3
! Server version: 2.2
=== Apps
casual-quacking
hidden-mongoose
zephyr-instinct
```

It would make it easier to use the output of `deis` commands as input to other commands if a `-q --quiet` flag was available which removed the superfluous output.