Support delta compression

[FZambia]

Introduce a way to set new Subscription delta option:

const sub = centrifuge.newSubscription(channel, {
  delta: 'fossil'
});

Option delta can only have fossil value now (delta compression based on Fossil algorithm).

When set, client will try to negotiate delta compression in channel with a server. If server allows this delta type – messages in channel MAY be delta-compressed. Whether messages are delta compressed or not depends on server publishing logic.