* Closes #8: Handling of optional values (#17)
* Refs #8: add optional and required fields to config node
* Refs #8: provide builder methods for optional and required access
* Refs #8: update readme
* Refs #8: Version to 1.1.1
* Fix badge link in readme
* Closes #19: Validation of optional fields throws errors when the field does not exist (#20)
* Refs #19: add tests
* Refs #19: skip None values in validation
* increment version
* Remove accidentally added dir (#21)
* Closes #24: Use proper python 3.7 for travis and cache pip dependencies (#27)
* Closes #25: Add new keys to the config (#28)
* Refs #25: add tests
* Refs #25: implement adding of new elements and add upsert flag
* update readme
* Refs #25: refactor output
* Closes #26: Merge with ENV variables (#29)
* Add documentation to config builder
* Refs #26: add test
* Refs #26: merge environment variables with config
* increment version to 1.1.3
* update readme