Implementing WebSub Client

[kwaa]

https://en.wikipedia.org/wiki/WebSub

Currently Hatsu periodically checks the user feed for updates, but it can be made more instantaneous with WebSub.

Considering that there is currently no suitable WebSub library in the Rust ecosystem, this will probably be delayed until I find one.

[kwaa]

Perhaps Hatsu users who have set up a WebSub hub for JSON / Atom Feed can leave a comment here so I can decide whether or not to do this feature based on the number of comments.