---------
* Change `tag_class` process. If the tag class value starts with `+` then
it'll combine the default class value. If not, tag class value will replace
the default class value. For example:
<code>
class=linenums
print 'hello, world'
</code>
will make pre class value `linenums`, but if you pass `tag_class = {'pre':'prettify'}`
to parseHtml() function, it'll overwrite the `linenums`, the result will be:
<pre class="prettify">
But if you pass `tag_class = {'pre':'+prettify'}`, the result will be:
<pre class="prettify linenums">