Add HTML table export format (--html)

Export article data in HTML table syntax.

```
./scholar.py --html -c 1 -a "Peter Norvig"

<table>
	<tr>
		<th>Title</th>
		<th>A modern approach</th>
	</tr>
	<tr>
		<td>Year</td>
		<td>1995</td>
	</tr>
	<tr>
		<td>Citations</td>
		<td>23127</td>
	</tr>
	<tr>
		<td>Versions</td>
		<td>60</td>
	</tr>
	<tr>
		<td>Cluster ID</td>
		<td>16379301881339907933</td>
	</tr>
	<tr>
		<td>Citations list</td>
		<td>http://scholar.google.com/scholar?cites=16379301881339907933&as_sdt=2005&sciodt=0,5&hl=en</td>
	</tr>
	<tr>
		<td>Versions list</td>
		<td>http://scholar.google.com/scholar?cluster=16379301881339907933&hl=en&as_sdt=0,5</td>
	</tr>
</table>
```

This text can also contain HTML, so using the previous code here we get:

<table>
	<tr>
		<th>Title</th>
		<th>A modern approach</th>
	</tr>
	<tr>
		<td>Year</td>
		<td>1995</td>
	</tr>
	<tr>
		<td>Citations</td>
		<td>23127</td>
	</tr>
	<tr>
		<td>Versions</td>
		<td>60</td>
	</tr>
	<tr>
		<td>Cluster ID</td>
		<td>16379301881339907933</td>
	</tr>
	<tr>
		<td>Citations list</td>
		<td>http://scholar.google.com/scholar?cites=16379301881339907933&as_sdt=2005&sciodt=0,5&hl=en</td>
	</tr>
	<tr>
		<td>Versions list</td>
		<td>http://scholar.google.com/scholar?cluster=16379301881339907933&hl=en&as_sdt=0,5</td>
	</tr>
</table>