Respect custom API URL for all endpoints

[muety]

muety/wakapi#153 brought up an issue related to the fact that many endpoints inside the CLI are hard-coded. It would be nice to be able to configure the base URL via wakatime.cfg instead, just like it's already possible for the heartbeats endpoint.

For instance, here: https://github.com/wakatime/wakatime/blob/e8deb156f1c2d26e5cf874da97f7b4354b3f5d20/wakatime/api.py#L173

[alanhamlett]

We'll do this in our new Go language version of wakatime-cli being released soon.

To do this, we need to change api_url to be the base api url instead of the heartbeats endpoint.

[muety]

Would you accept a PR for this, even though the new CLI is on its way? If so, I'll have a look.