gherkin-streams: accept an optional `relativeTo` path for uris (#1510)
gherkin-streams: accept an optional `relativeTo` path for uris (#1510)

* add a failing test to get started

* make it pass

* refactor into new options object

* improve testing a bit

* update CHANGELOG.md

* update documentation

* Update README.md
cucumber/gherkin-javascript,cucumber/gherkin-javascript