Refactor browserify system

Instead of opting to replace when compiling to the distribution
files, this refactoring uses the `package.json`s `browser`
field. This ensures other tools adhering to that `browser`
field work.

Related to GH-1.

package.json

@@ -21,6 +21,9 @@
     "vfile": "^1.0.0",
     "ware": "^1.3.0"
   },
+  "browser": {
+    "node-extend": "extend"
+  },
   "repository": {
     "type": "git",
     "url": "https://github.com/wooorm/unified.git"
@@ -56,7 +59,7 @@
     "lint": "npm run lint-api && npm run lint-style",
     "make": "npm run lint && npm run test-coverage",
     "build-md": "mdast . --quiet",
-    "build-bundle": "browserify index.js -s AttachWare -u node-extend > unified.js",
+    "build-bundle": "browserify index.js -s AttachWare > unified.js",
     "postbuild-bundle": "esmangle unified.js > unified.min.js",
     "build": "npm run build-md && npm run build-bundle"
   }

unified.js

@@ -292,7 +292,7 @@ function unified(options) {
 
 module.exports = unified;
 
-},{"attach-ware":2,"bail":3,"extend":8,"node-extend":undefined,"unherit":12,"vfile":13,"ware":14}],2:[function(require,module,exports){
+},{"attach-ware":2,"bail":3,"extend":8,"node-extend":8,"unherit":12,"vfile":13,"ware":14}],2:[function(require,module,exports){
 /**
  * @author Titus Wormer
  * @copyright 2015 Titus Wormer
@@ -2983,6 +2983,55 @@ try {
     SEPARATOR = require('pa' + 'th').sep;
 } catch (e) { /* empty */ }
 
+/**
+ * Construct a new file message.
+ *
+ * Note: We cannot invoke `Error` on the created context,
+ * as that adds readonly `line` and `column` attributes on
+ * Safari 9, thus throwing and failing the data.
+ *
+ * @example
+ *   var message = new VFileMessage('Whoops!');
+ *
+ *   message instanceof Error // true
+ *
+ * @constructor
+ * @class {VFileMessage}
+ * @param {string} reason - Reason for messaging.
+ * @property {boolean} [fatal=null] - Whether the message
+ *   is fatal.
+ * @property {string} [name=''] - File-name and positional
+ *   information.
+ * @property {string} [file=''] - File-path.
+ * @property {string} [reason=''] - Reason for messaging.
+ * @property {number} [line=null] - Start of message.
+ * @property {number} [column=null] - Start of message.
+ * @property {Position|Location} [location=null] - Place of
+ *   message.
+ * @property {string} [stack] - Stack-trace of warning.
+ */
+function VFileMessage(reason) {
+    this.message = reason;
+}
+
+/**
+ * Inherit from `Error#`.
+ */
+function VFileMessagePrototype() {}
+
+VFileMessagePrototype.prototype = Error.prototype;
+
+var proto = new VFileMessagePrototype();
+
+VFileMessage.prototype = proto;
+
+/*
+ * Expose defaults.
+ */
+
+proto.file = proto.name = proto.reason = proto.message = proto.stack = '';
+proto.fatal = proto.column = proto.line = null;
+
 /**
  * File-related message with location information.
  *
@@ -3310,7 +3359,7 @@ function message(reason, position) {
         }
     }
 
-    err = new Error(reason.message || reason);
+    err = new VFileMessage(reason.message || reason);
 
     err.name = (filePath ? filePath + ':' : '') + range;
     err.file = filePath;