Allow option for using WinNFSD as sync folder provider on Windows

[henkler]

This speeds up Sage development on Windows with Virtualbox significantly (at least an order of magnitude in my case). Virtualbox synced folders are incredibly slow with large numbers of files on Windows.

By default, image will still use standard synced folders on Windows unless vagrant-winnfsd is actually installed. Standard sync should probably stay the default for now as WinNFSD isn't being actively maintained anymore, and there doesn't seem to be a good replacement yet.

That being said, the performance boost is incredible after the switch.

To switch to using WinNFSd:

vagrant halt
vagrant plugin install vagrant-winnfsd
vagrant plugin install vagrant-bindfs
vagrant up

Note on usage: 'vagrant up' must be run with Admin privileges the first time it is run for it to set up the exports correctly. Subsequent calls to vagrant up do not appear to need Admin privileges. Tested on Windows 10.

[swalkinshaw]

Looks simple enough 👍

Would you be able to add a section about this to our Windows docs?

[henkler]

Just put in a PR for the Windows docs. Thanks!