Add support for <u> to emphasis

Closes GH-22.

Author: wooorm

lib/handlers/index.js

@@ -28,7 +28,7 @@ exports.tr = require('./table-row');
 exports.th = exports.td = require('./table-cell');
 exports.li = require('./list-item');
 exports.strong = exports.b = require('./strong');
-exports.em = exports.i = require('./emphasis');
+exports.em = exports.i = exports.u = require('./emphasis');
 exports.del = exports.s = exports.strike = require('./delete');
 exports.code = exports.kbd = exports.samp = exports.tt = exports.var = require('./inline-code');
 exports.img = require('./image');

test/fixtures/u/index.html

@@ -0,0 +1 @@
+<p><u>Hello World</u>.</p>

test/fixtures/u/index.json

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

test/fixtures/u/index.md

@@ -0,0 +1 @@
+_Hello World_.