Add a command to update all the recipes repo added

[liesrock]

I don't know if there is a way to the the above with a command at the moment: it can be useful to avoid to manually "git pull" each of the recipes repos added.

[MarcoRuzzon]

At the moment we support only one recipes repository, if you try to add a second one before removing the first you should get an error. We are planning to add support for multiple ones and when we do it a command like forest add-recipes update could do the trick.

[liesrock]

Ok thanks, @damigas fyi