Export the client endpoint

[aerostitch]

Hi,

Thanks for providing that library. To ease the usage and the testing it would be good to allow your users to update the endpoint after the client initialization.

This would allow me at least to avoid having to use global variables instead of constants or having to expose in my client the API URL that is not changing except in the tests. https://github.com/VEVO/slab-go/blob/master/slab/slab_test.go#L32

As endpoint is currently unexported that won't break any dependent codes either.

Thanks for your help, Joseph