[Feature]: rocks update

[vhyrro]

rocks update would simply take the current rock tree (or the one specified under --tree) and update all rocks in a multithreaded fashion.

I'm trying to think ahead here and also consider Lua projects - rocks update should always consider the current project before it tries to update all rocks in the user-local tree. Perhaps this could be extended even further, maybe rocks should set the default tree to the current project's tree if it detects it's in a lua project.