WIP: Add Vagrant configuration

Provides a ready-to-go environment for working on Julia, with an eye towards Windows developers who would like to contribute to core, or to test on Linux. The Julia repository is shared between the host and guest, so editing files can be done in the host environment in your editor of choice.

It's a bit clunky--you have to run `git submodule init` and `git submodule update` from the host before our Make process gets to it or it messes up the submodule references; you also need to use the `make $BUILDOPTS` (aliased to `jlmake`) to ensure you get a fully USE_SYSTEM_* build. At the very least, a full build in VirtualBox on Windows is impossible due to the `aux/` directory in the libunwind tarball.

Due to the limitations of VirtualBox shared folders, symlinks need to be replaced with either file copies or elided entirely.