Multi-context support WIP

@isaacs @piscisaureus @trevnorris and anyone else who wants to look.

There is currently no JS API but you can start scripts in a separate context with `--context='path/to/script.js arg1 arg2 etc'`.  The security token defaults to off and can be set with `--security-token=<token>`, which then affects the --context arguments that follow it.  (You can't turn it off again currently.)

There is still some cleanup to do here and there but it's mostly complete.  I have half a mind to eradicate node_isolate while I'm here but there's almost 1,000 use sites...

@nathansobo  @zcbenz Feedback welcome.