Add extra_sources option and allow to add hashes as a settings source

[romanoji]

Hey! I've added new feature, which allows to define extra config sources before config load. I think this feature will be useful, because starting from this version config validation feature will be available. Unfortunately, validation doesn't work well, when you want to add new config files - it's impossible to validate values that appear in the files added at the runtime (using Settings.add_source!).

This may be also a sort of remedy for railsconfig/config#142.

[kamilpp]

👍

[pkuczynski]

We could have a look at this idea @romanoji but you would have to solve conflicts first.

[cjlarose]

I'm going to close this as inactive. The core issue is still valid: it's not particularly easy to defer validation of the config object to after your application has had the chance to add all of its config sources.