Last release with embedded contrib directory

This release marks the last point at which the contrib directory is included as part of the principal gofpdf project. The packages included in the contrib directory are those that depend on other packages that are not part of Go's standard library. These contributed packages will be moved to a new repository (github.com/jung-kurt/gofpdfcontrib). This will allow the Go module file for the principal gofpdf package to be free of any non-standard library dependency.