feat: add support for dynamic namespaces (WIP)

[darrachequesne]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

New behaviour

The following test should describe the expected behaviour:

const socket = client(srv, '/dynamic-101');
io.of(/^\/dynamic-\d+$/).on('connect', (socket) => {
  // socket.nsp.name = '/dynamic-101'
});

socket.on('error', function(err) {
  expect().fail();
});
socket.on('connect', () => {
  // ...
});

Other information (e.g. related issues)

[xaqbr]

Is there a way to pass data into next when using a function to create dynamic namespaces?