duo: Use win-fork rather than child_process.spawn

See:

 * https://github.com/duojs/duo/pull/118#discussion_r15720060
* https://github.com/duojs/duo/issues/221
deps: Replace win-fork with cross-spawn
Merge pull request #223 from stephenmathieson/add/windows-subcommand-support

duo: Use win-fork rather than child_process.spawn