Support for Nested Dependencies

I've been using this library for several of my projects.  One of the projects is a shared library.  I encountered a bug where when a nest dependency *and* the parent project both use `rekuire`, NPM decides to be "helpful" and only loads `rekuire` at the root level.

This causes the nested library to no longer work, since it's ignored by the root level's rekuire.  I fixed this by allowing multiple directories to be specified for scanning.  I've also updated the README with instructions for usage.

I ran the tests.  3 of them are failing (at least on my machine).  I verified that those 3 tests were failing prior to any of my modifications.