feat(serve-static): add rewriteRequestPath option

[yusukebe]

This PR enables rewriting a request path for Serve Static middleware.

If you want to serve files in ./.foojs with the request path /__foo/*, you can write:

 app.use(
  '/__foo/*',
  serveStatic({
    root: './.foojs/',
    rewriteRequestPath: (path: string) => path.replace(/^\/__foo/, ''),
  })
)

Fix #29