[Feature] Ability to set url before reconnecting

Right now in order to change websocket url you have to create a new instance of Sockette. This an issue for us because our url includes authorization token which expires after some time and when Sockette tries to reconnect it tries to reconnect using an expired token.

Here are a couple of possible solutions to this issue:

1. getUrl config which can be set to a function which either returns a string directly or a Promise<string>. Sockette would use this function every time it tries to automatically establish websocket connection.
2. setUrl() method on the Sockette class and ability to set onreconnect config to a function which returns a Promise. Sockette would wait for onreconnect promise to finish before it would try to reestablish connection.

@lukeed what do you think about it? Do you have any better suggestions?

Start by reading Sockette's reconnect path and how it currently stores and uses the WebSocket URL. Compare the proposed getUrl and setUrl/onreconnect approaches, including asynchronous reconnect behavior, and confirm the intended API with the maintainer before changing anything. Done means reconnects can obtain a fresh authorization URL without creating a new Sockette instance, with coverage for the selected behavior.