Allow stdin to be a file URL

[ehmicky]

Part of #592.

This allows the stdin option to be a file URL. I will send another PR for stdout/stderr.

This is an alternative implementation: see #614 for using file path strings instead. Should we allow file URLs, file path strings, or both?

• File path strings are easier for users.
• File URLs are more "proper" as they are typed and work in browsers.
• Allowing both is what most Node.js core methods do.

[sindresorhus]

We should support both.