markdown: Fix panic: assignment to entry in nil map

Ensures metadata.Variables is made

middleware/markdown/metadata.go

@@ -12,9 +12,9 @@ import (
 
 var (
 	parsers = []MetadataParser{
-		&JSONMetadataParser{},
-		&TOMLMetadataParser{},
-		&YAMLMetadataParser{},
+		&JSONMetadataParser{metadata: Metadata{Variables: make(map[string]interface{})}},
+		&TOMLMetadataParser{metadata: Metadata{Variables: make(map[string]interface{})}},
+		&YAMLMetadataParser{metadata: Metadata{Variables: make(map[string]interface{})}},
 	}
 )
 

middleware/markdown/process.go

@@ -21,7 +21,7 @@ const (
 // process processes the contents of a page.
 // It parses the metadata (if any) and uses the template (if found)
 func (md Markdown) process(c Config, requestPath string, b []byte) ([]byte, error) {
-	var metadata = Metadata{}
+	var metadata = Metadata{Variables: make(map[string]interface{})}
 	var markdown []byte
 	var err error