Adding ctx to public methods

[yurifedoseev]

Hi @dukex, thanks for your work!
I'd like to add context.Context objects to public methods - to have a better control on requests and cancelling logic. Otherwise I have no control on cancelling hanged request from my application code.
Before creating a PR I want to discuss it here. I want to change existing public method. For example it will be. Track(ctx, distinctId, eventName string, e *Event). Even it's a breaking change it can be fixed via context.TODO() if user doesnt have any other ctx to use.
Also I can create another sets of methods like TrackCtx but it will make api and the code a little bit messy.

What do you think?

[yurifedoseev]

Did a pr #16