Add plugin to cleanup XML namespaces

Added plugin to cleanup (remove unused and minify used) XML namespaces. Replaced 'removeUnusedNS' plugin with new plugin ('cleanupNS'). Fixed bug in 'cleanupIDs' which would allow a generated minified ID to be the same as a preserved ID.

.svgo.yml

@@ -49,7 +49,7 @@ plugins:
   - removeEmptyAttrs
   - removeEmptyContainers
   - mergePaths
-  - removeUnusedNS
+  - cleanupNS
   - sortAttrs
   - removeTitle
   - removeDesc

plugins/cleanupIDs.js

@@ -29,14 +29,16 @@ var referencesProps = new Set(require('./_collections').referencesProps),
  * Remove unused and minify used IDs
  * (only if there are no any <style> or <script>).
  *
- * @param {Object} item current iteration item
+ * @param {Object} data full AST
  * @param {Object} params plugin params
+ * @return {Object} full AST
  *
  * @author Kir Belevich
  */
 exports.fn = function(data, params) {
     var currentID,
         currentIDstring,
+        currentIDmap,
         IDs = new Map(),
         referencesIDs = new Map(),
         hasStyleOrScript = false,
@@ -125,7 +127,10 @@ exports.fn = function(data, params) {
         if (IDs.has(key)) {
             // replace referenced IDs with the minified ones
             if (params.minify && !preserveIDs.has(key)) {
-                currentIDstring = getIDstring(currentID = generateID(currentID), params);
+                currentIDmap = generateID(currentID, preserveIDs, params);
+                currentID = currentIDmap.get('arr');
+                currentIDstring = currentIDmap.get('str');
+
                 IDs.get(key).attr('id').value = currentIDstring;
 
                 for (var attr of ref[1]) {
@@ -153,10 +158,12 @@ exports.fn = function(data, params) {
  * Generate unique minimal ID.
  *
  * @param {Array} [currentID] current ID
- * @return {Array} generated ID array
+ * @param {Set} preserveIDs IDs to be preserved
+ * @param {Object} params plugin params
+ * @return {Map} generated ID array and string
  */
-function generateID(currentID) {
-    if (!currentID) return [0];
+function generateID(currentID, preserveIDs, params) {
+    if (!currentID) currentID = [-1];
 
     currentID[currentID.length - 1]++;
 
@@ -173,13 +180,24 @@ function generateID(currentID) {
         currentID[0] = 0;
         currentID.unshift(0);
     }
-    return currentID;
+
+    var currentIDstring = getIDstring(currentID, params),
+        currentIDmap = new Map();
+
+    currentIDmap.set('arr', currentID);
+    currentIDmap.set('str', currentIDstring);
+
+    if (preserveIDs.size && preserveIDs.has(currentIDstring)) {
+        currentIDmap = generateID(currentID, preserveIDs, params);
+    }
+    return currentIDmap;
 }
 
 /**
  * Get string from generated ID array.
  *
  * @param {Array} arr input ID array
+ * @param {Object} params plugin params
  * @return {String} output ID string
  */
 function getIDstring(arr, params) {