HTML reader: Allow space between <col> and </col>.

Test case:

```
<table border="1">
  <colgroup>
    <col> </col>
    <col></col>
  </colgroup>
  <tbody>
    <tr>
        <td>X</td>
        <td>Y</td>
    </tr>
    <tr>
        <td>1</td>
        <td>2</td>
    </tr>
  </tbody>
</table>

```

src/Text/Pandoc/Readers/HTML.hs

@@ -262,6 +262,7 @@ pTable = try $ do
 pCol :: TagParser Double
 pCol = try $ do
   TagOpen _ attribs <- pSatisfy (~== TagOpen "col" [])
+  skipMany pBlank
   optional $ pSatisfy (~== TagClose "col")
   skipMany pBlank
   return $ case lookup "width" attribs of