Where to save user data

[jbasko]

Config instance may want to be aware of where to save user changes when asked to.

See confuse library.

[jbasko]

We now have load_sources setting. Maybe load_stores is better and then have dump_stores or write_stores?

Be mindful:

• if you have a config tree of structure A(B(C)) then A.sources have precedence over B.sources and C.sources in loading values.
• if you now make changes and want to save them, then where you will write them to depends on where you call writer from. If you call it from A then you can only reliably write to A.sources. Because B and C could be configs for some sub-component which should be runnable/configurable independently from A.
• If A has no sources then you should write changes on the children Configs.