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* Support for multiple domains within --domain-filter

The parameter accepts a comma separated list of domains with or without trailing dot. Example: --domain-filter="example.org, company.test.,staging.com". Closes #247 and #229

* Add boilerplate header

* Add documentation for methods and structs

* use StringsVar for the domain-filter flag

* go fmt

* Remove camel case from tests

* Revert changes in README.md

* Move DomainFilter to provider package

* Make a new slice and copy elements to it

* Update CHANGELOG.md

* docs: change minor spelling mistake