Add `--indepent` option

[egoist]

For example to test against node 4/5/7, it copies the project and creates 3 folder to run each version, before running it executes git clean -xfd to remove unwanted files and runs npm install then

[balupton]

This can make a lot of things no longer work, as they may be dependent on the specific path location. It would be better for people to run with the serial flag, and then use the pretest and posttest npm script hooks to prepare and cleanup.