`rb each` command for running a command across all installed rubies

This command work much like `rb exec` except it runs the given cmd,
not against a _given_ rubie, but against _all installed_ rubies.

The particular use-case I have in mind for this cmd is for running
a build against multiple rubies.
Merge pull request #39 from redding/kr-exec-all

`rb each` command for running a command across all installed rubies