Support variable at address start

[bookmoons]

Supports variables at address start:

{{protocol}}://example.com
{{base}}/api
{{address}}

Defaults protocol at runtime if necessary. Defaults to http to follow Postman app behavior.

Closes #27. Closes #30.

Thanks to @mintbridge for the report and initial good work toward a solution.

[robingustafsson]

Thanks @bookmoons, LGTM. Merging.