Improve the customizable HttpClient configuration

[ChrisKruegerDev]

Related to #25

Idea:

        val tmdb = Tmdb3 {
            tmdbApiKey = "key"
            tmdbAuthenticationToken = "token"

            tmdbAccountCredentials {
                val storage = TmdbAccountStorage() // use own class here
                sessionId { storage.sessionId  }
                requestToken { storage.requestToken }
                accessToken { storage.accessToken }
            }

            httpClient(OkHttp) {
               engine {
                    // for own client
               }
            }

            httpClient {
                // for custom config
            }
        }

[chrisbanes]

+1 for making these lazy. You can probably make this function inline

[chrisbanes]

Yeah this is tricky because you need a File