Add Support for project-level Config Files

[theAkito]

Basically, project_api_key, but with api_url. For example, project_api_url.

The above is the simple & quick solution for one bigger problem. The ideal solution for all problems on the other hand would be each project having its own optional configuration file, overwriting settings from the global file. So, if the project file has a different api_url only that one gets overriden.

We already may use a .wakatime-project, so why not actually making it work (or simply adding a normal .wakatime in the project directory)?