WIP: Allow region filtering

[remyleone]

This is an answer to #446
This PR adds an example showing scw ps with a filter.

Should we use the --region flag passed before the command ps to filter the API on which the ps commands run upon?

[moul]

LGTM 👍

[remyleone]

@moul What about the --region question? It seems that this questions come back often "region" tag issues.

[moul]

Should we use the --region flag passed before the command ps to filter the API on which the ps commands run upon?

By default, I'm more for using this as a global variable as its (or will be) useful for the majority of the commands in scw.

But from what I recently saw, it's more and more standard to put every argument at the end of the command, probably "more POSIX," i.e., https://github.com/spf13/cobra

Keep in mind that the current scw API is based on docker's one which makes the distinction between global and command-specific flags

I suggest you vote with the rest of the team to choose a norm