Adding Option to give users finer control of what results get cached.

[ygonzal6]

Background information
Our team is currently using the Funnel to cache network request.
It was noticed that error responses are also cached, which is less then ideal.

Implementation summary
Add an Option to the Funnel constructor that takes in a predicate that will return true if a result
should be cached and false otherwise. The predicate will have the following signature func(interface{}, error)bool
which matches the return value from the execute.
With this solutions user can chose to have complete control of what results get cached or whether to use the default behavior.

Before & after
Before:
No option to control what gets cached.

After:
func WithShouldCachePredicate(p func(interface{}, error) bool) Option {
return func(cfg *Config) {
cfg.shouldCache = p
}
}

Risk mitigation
Unit test has been added to cover the new Option

Ticket
https://jira.mailchimp.com/browse/DATABRIDGE-1003

[ollana]

This is great, thank you for the contribution!
do you mind adding a small usage example to the documentation?