Add a "rehash" command to ivy

[jasonewall]

Similar to rbenv's rehash - since it also finds bin commands from installed gems and creates shims for them - making them all available for the given ruby version. In contrast jenv does not (nor should it) do this because jars are not installed as part of the java install - so this is pretty far outside of jenv's purview.

[jasonewall]

Step 1) Where does ivy dump jars on a ivy install invocation, and how can we detect which binaries came with it?

First challenge - doesn't appear that installing artifacts through maven/ivy comes with the shell commands that we could directly link to using a file system scan - similar to how (I expect) rbenv/jenv's rehash commands would work.

So... what do?

[jasonewall]

This ain't gonna work without some kind of agreed on registry/way to publish bin commands with maven downloadable assets.

[jasonewall]

Just making it so executable packages can be dropped in a folder after being manually downloaded.

[jasonewall]

Sort of works... copy an uncompressed package to ~/.ivy-cli/packages and call ivy rehash - everything in the ./bin directory of the package is now in your path automatically.

Doesn't handle multiple versions yet