Support virtual path prefix.

[AndersDJohnson]

No description provided.

[scottcorgan]

Can you elaborate just a bit more? Not sure I fully understand.

[AndersDJohnson]

@scottcorgan A virtual path prefix would be an optional argument that instead of serving files at the root path / under the given local port, e.g. /index.html, it would instead serve under a virtual sub-path that doesn't exist in the file system e.g. /sub/path/index.html if you passed e.g. --path=sub/path.