Add support for downloading source artifacts from Github release

This adds support for retrieving the tarball or zip source artifact from a
Github release. This is done through defining a
"params.include_source_tarball" or "params.include_source_zip" boolean setting.

With this, the resource will download the respective source artfact into the
target directory as either "source.tar.gz" or "source.zip".

resources.go

@@ -20,7 +20,9 @@ type InRequest struct {
 }
 
 type InParams struct {
-	Globs []string `json:"globs"`
+	Globs                []string `json:"globs"`
+	IncludeSourceTarball bool     `json:"include_source_tarball"`
+	IncludeSourceZip     bool     `json:"include_source_zip"`
 }
 
 type InResponse struct {