Make api url configurable somehow

[ChrisTheBaron]

https://github.com/UniversityRadioYork/myradio-go/blob/master/api.go#L20

func NewSession(apikey string) (*Session, error) {
    url, err := url.Parse(`https://ury.york.ac.uk/api/v2`)
    if err != nil {
        return nil, err
    }
...

[MattWindsor91]

I agree, and was thinking about this myself when working on #35.

What would be the best way of doing this, except for moving the need to supply the URL to the user eg.

func NewSession(url url.URL, apikey string) { /* ... */ }

and maybe providing something like

var MyRadioURL = url.URL{ /* ... */ }

?