new plugin: add classes to SVG element

.svgo.yml

@@ -47,4 +47,5 @@ plugins:
   - transformsWithOnePath
   - sortAttrs
   - removeTitle
-  - removeDesc
+  - removeDesc
+  - addClassesToSVGElement

plugins/addClassesToSVGElement.js

@@ -0,0 +1,38 @@
+'use strict';
+
+exports.type = 'full';
+
+exports.active = true;
+
+/**
+ * Add classnames to an outer <svg> element.
+ *
+ * @author April Arcus
+ */
+exports.fn = function(data, params) {
+
+    var classNames = params.classNames || [ params.className ]
+    var svg = data.content[0]
+
+    if (svg.isElem('svg')) {
+
+        if (svg.hasAttr('class')) {
+            svg.attr('class').value =
+                svg.attr('class').value
+                    .split(' ')
+                    .concat(classNames)
+                    .join(' ');
+        } else {
+            svg.addAttr({
+                name: 'class',
+                value: classNames.join(' '),
+                prefix: '',
+                local: 'class'
+            });
+        }
+
+    }
+
+    return data;
+
+};