colspan is entirely ignored, which results in misformated tables

For example this test:
`echo 'AB123' | html2text --pad-tables`

Output is currently weirdly mangled with incorrect columns:
```
A | B
----|----
1 | 2 | 3
```

Today I learned: markdown bizarrely has no defined way of dealing with colspan or rowspan. The correct syntax is undefined here, but adding a blank dummy cell seems to be the best option at the moment, so the output should be:

```
A | | B
---|---|---
1 | 2 | 3
```

Or as shown here in github's implementation of markdown:

A | | B
---|---|---
1 | 2 | 3