Flexible cmd_vel_mux reconfiguration

[corot]

Right now we use a fixed configuration. No good for someone who wants to do something different.

Possible approaches:

• Force apps to do dynamic reconfigure in every app.
• Write our own node to request current configuration, save it, send new configuration and then spin. Send the old configuration on shutdown. Add these to the app launchers, this will have the lifetime of the app.

@stonier - I really like the latter solution. It doesn't force 99 out of 100 users to do extra work for an esoteric use case.

@jorge was thinking of rolling back to 'default' states instead of saving it. But then considered two apps on top of each other, the first rollback should not be to the default, but to the first app's configuration.

Do this after 2.0 release.