Support observer hooks

[h2non]

Replace events in favor of observer hooks with control flow capabilities.

const client = resilient({})

client.observe('before request', function (ctx, next) {
 // logic goes here...  
})

client.observe('dialing', function (ctx, next) {
 // logic goes here...  
})

client.observe('retry', function (ctx, next) {
 // logic goes here...  
})

client.observe('response', function (ctx, next) {
 // logic goes here...  
})