Whitelisted th and td to fix #79

``HTML_SANITIZE_OUTPUT`` breaks alignments in tables.

So, the input with ``HTML_SANITIZE_OUTPUT`` flag:
```
| Tables   |      Are      |  Cool |
|----------|:-------------:|------:|
| col 1 is |  left-aligned | $1600 |
```
will lead to the result:
```
<table>
	<thead>
		<tr>
			<th>Tables</th>
			<th>Are</th>
			<th>Cool</th>
		</tr>
	</thead>
	<tbody>
		<tr>
			<td>col 1 is</td>
			<td>left-aligned</td>
			<td>$1600</td>
		</tr>
	</tbody>
</table>
```
instead of:
```
<table>
	<thead>
		<tr>
			<th>Tables</th>
			<th align="center">Are</th>
			<th align="right">Cool</th>
		</tr>
	</thead>
	<tbody>
		<tr>
			<td>col 1 is</td>
			<td align="center">left-aligned</td>
			<td align="right">$1600</td>
		</tr>
	</tbody>
</table>
```

As @mprobst [pointed out](https://github.com/russross/blackfriday/issues/79#issuecomment-49731427) it can be fixed by white listing "td: align". Moreover, "th: align" should be white listed as well.