Test correctness of configuration keys (and maybe types?)

[SergioLuis]

In PR #19 basic configuration was implemented. However, the configuration system is really decoupled. You need to:

1. Define the correct configuration key, with the correct type, in package.json
2. Define the correct member in the Config interface.

This process is quite error-prone. Complete tests in config.test.ts to:

• Check that every contributes.configuration key in package.json starts with plastic-scm.
• Check that every contributes.configuration key exists in the Config interface.
• Check that every property defined in the Config interface exists under contributes.configuration

If its possible, it would be extremely nice to...

• Check that every configuration key in package.json defines the same type used later on the Config interface.