Add VirtualBox shared folders creation

This includes three new commandline flags to control this behavior, for those who don't want the default (which is currently automount `/Users` for OS X, and automount `C:\Users` for Windows, but will change to automount `homeDir()` at some point in the future):

- `--no-vbox-share=false: Disable VirtualBox share creation during 'init'`
- `--vbox-share="": Change VirtualBox shared folder created during 'init' from the default autodetection logic`
- `--vbox-share-name="": Change VirtualBox shared folder name created during 'init' from the default of being automatically determined based on the share directory path`
That's fair, although can't you use a boolean flag like `--no-vbox-share` directly (ie, no `=true`, since that's implied)?  Also, we already have `--no-dummy="": Example parameter for the dummy driver.`, so this isn't without precedent.