Add support for <wbr>

Closes GH-21.

Author: wooorm

lib/handlers/index.js

@@ -46,6 +46,8 @@ exports.h6 = require('./heading');
 
 exports.a = require('./link');
 
+exports.wbr = require('./wbr');
+
 function ignore() {}
 
 function children(h, node) {

lib/handlers/wbr.js

@@ -0,0 +1,7 @@
+'use strict';
+
+module.exports = zws;
+
+function zws(h, node) {
+  return h.augment(node, {type: 'text', value: '\u200B'});
+}

test/fixtures/wbr/index.html

@@ -0,0 +1,2 @@
+<p>al<wbr>pha</p>
+<p><wbr></p>

test/fixtures/wbr/index.json

@@ -0,0 +1,3 @@
+{
+  "fragment": true
+}

test/fixtures/wbr/index.md

@@ -0,0 +1,3 @@
+al​pha
+
+​