cmd.go: Nicer error for unknown command

This may not be that interesting for experienced tsuru users and developers but I think that it makes tsuru friendlier for beginners.

This is modeled after the output that `git` gives in this situation (since `git` is renowned for user friendliness :smile:).

E.g.:

    [marca@marca-mac2 tsuru-client]$ ~/go/bin/tsuru target
    /Users/marca/go/bin/tsuru: "target" is not a tsuru command. See "tsuru help".

    Did you mean?
            target-add
            target-remove
            target-set
            target-list

    [marca@marca-mac2 tsuru-client]$ ~/go/bin/tsuru team
    /Users/marca/go/bin/tsuru: "team" is not a tsuru command. See "tsuru help".

    Did you mean?
            team-user-add
            team-remove
            team-create
            team-list
            team-user-list
            team-user-remove