Improve passing data to forked processes

[sindresorhus]

From: #296 (comment)

Currently we just proxy the CLI args manually. We should instead just JSON.stringify all the data we want to pass and pass it as the only argument.

[vadimdemedes]

Or we could just pass process.argv.slice(2) to the fork process?

[sindresorhus]

@vdemedes Sure, but now all the places we use those things have to do an additional argument parsing. Even worse when we want to send arrays and more complex data structures down to the child process, like with the --require PR. Would be much nicer if anywhere we wanted to use the info from the parent process we could just JSON.parse(process.argv[2]).

[vadimdemedes]

Either way is good for me ;)