Re-export uc.micro and mdurl

lib/common/utils.js

@@ -254,6 +254,13 @@ function normalizeReference(str) {
 
 ////////////////////////////////////////////////////////////////////////////////
 
+// Re-export libraries commonly used in both markdown-it and its plugins,
+// so plugins won't have to depend on them explicitly, which reduces their
+// bundled size (e.g. a browser build).
+//
+exports.lib                 = {};
+exports.lib.mdurl           = require('mdurl');
+
 exports.assign              = assign;
 exports.isString            = isString;
 exports.has                 = has;