docs: markdown tabs are hard

refactor: removed unnecessary exports

The type `Source` was removed since it was adding complication with no
real benefit. All of the implementations and documentation was updated
to reflect the change.

errors: add error types for common errors

refactor: no need to export `envOption` type

docs: comments for `DefaultLoadOptions` and `LoadConfig`

docs: added badge for go report card

init: initial commit; some additional test coverage needed