Port all non documentation commits from typesafe/config

This issue should remain open as a reminder to periodically review commits and port the issue across. If you find a new PR/commit, please open a new issue to handle that commit. Last `lightbend/config` commit is 1fc2aff4, https://github.com/lightbend/config/pull/597 (Nov 4, 2018)

1. Skip PR number if a doc or version update that is not needed.
2. Github uses sequential numbers for both Issues and PRs.
3. Some older sequenced PRs may have been merged after that date.

First PR is

1. - [x] https://github.com/lightbend/config/pull/568
Fixed via https://github.com/ekrich/sconfig/pull/460
3. - [ ] https://github.com/lightbend/config/pull/619
This relies on classloaders and is very JVM only.
5. - [ ] https://github.com/lightbend/config/pull/620
Env vars apply to both JVM and Scala Native so we need to see if this can coexist with Scala.js
6. - [x] https://github.com/lightbend/config/pull/648Fixed via https://github.com/ekrich/sconfig/pull/461
7. - [x] https://github.com/lightbend/config/pull/663
8. - [ ] https://github.com/lightbend/config/pull/686
This needs overrides 620 above first
10. - [x] https://github.com/lightbend/config/pull/701
This is resolved.
11. - [ ] https://github.com/lightbend/config/pull/708
For now wait on 708 and 709
12. - [x] https://github.com/lightbend/config/pull/709
`ConfigFactory` `public static java.util.Optional parseApplicationReplacement(ConfigParseOptions parseOptions)` is not ported to this library.
14. - [x] https://github.com/lightbend/config/pull/731Not found on inspection.
15. - [x] https://github.com/lightbend/config/pull/758
Must have been fixed prior to this list
17. - [x] https://github.com/lightbend/config/pull/798
Fixed via https://github.com/ekrich/sconfig/pull/458
19. - [x] https://github.com/lightbend/config/pull/817
Fixed via https://github.com/ekrich/sconfig/pull/459
20. - [ ] https://github.com/lightbend/config/pull/833
21. - [ ] https://github.com/lightbend/config/pull/839/

Review the unchecked upstream PRs listed in the issue, especially 619, 620, 686, 708, 833, and 839, and compare each with the current sconfig implementation. Start by determining which changes apply across the JVM, Scala.js, and Scala Native targets. Done means each applicable commit has been ported or has a separate issue documenting why it is not needed.

Written by the indexing model from the issue text.