Allow creating retrier with a function

[medakk]

This PR allows you to create a Retrier without creating a struct to satisfy the interface, just a function. Similar to http.Handler and http.HandlerFunc

Example:

linearRetrier := NewRetrierFunc(func(retry int) time.Duration {
    if retry <= 0 {
        return 0 * time.Millisecond
    }
    return time.Duration(retry) * time.Millisecond
})
client := heimdall.NewHTTPClient(timeout)
client.SetRetrier(retrier)

[rShetty]

@medakk Can you add this to docs too ?

[medakk]

Comments for go doc are present. Do you mean the project's README.md?

[rShetty]

@medakk Yes, README

[medakk]

Doc has been added.

[rShetty]

@medakk Thanks will merge.