add custom command ability

[makapuf]

It would be nice to add the ability to run any (git) command on every projects of the workspace.
My main use case here would be to check the sha1 status of each of the projects (by issuing git rev-parse HEAD for each repo) but there can be other uses.

[emlun]

This has been discussed before (#31)... but I might reconsider given the now doubled demand for it. :)

[makapuf]

After reading thw reasons it was closed I can understand. Another possibility would be to add sha1 in the status but I'd say this run or foreach command could be useful and not that simple to implement with a oneliner of you consider the case of deeper git directories or non git dirs besides git dirs . I can also think of gws do -- make