Fix go mod definition with major version bump

This release fixes the issue with the semantic version breaking bump of goquery to version 2.0

The go mod file has been updated and all internal imports now reference the require /v2 library suffix.

Examples are updated as well, with the new goquery import as:

"github.com/AbGuthrie/goquery/v2"
"github.com/AbGuthrie/goquery/v2/models"

Issues fixed:

Fix go mod and import paths for v2 release #123