Turn on config (+ final tweaks)

[jdegoes]

1. Moved Secret into the companion object of Config to avoid polluting the top-level name space
2. Improved the featureset of Secret to make it more amenable for storing sensitive information
3. Added a test suite for Config
4. Removed package private modifier on Config and ConfigProvider
5. Make all operators on Config trait by-name, unless they are functions

[jdegoes]

@adamgfraser Please remind me (if I forget) we need to turn back on MIMA checks for these classes, but based on 2.0.4.