GFM Tables are not parsed correctly when fewer than three hyphens exist in the delimeter row.

The [GFM Spec](https://github.github.com/gfm/#tables-extension-) allows a table's delimeter row to have a single hyphen per column.

The following code:

```markdown
|a|
|-|
|b|
```

is valid table:

|a|
|-|
|b|

But the `GFMFlavourDescriptor` just parses it as text:

```
Markdown:PARAGRAPH
Markdown:TEXT '|a|'
Markdown:EOL
Markdown:TEXT '|-|'
Markdown:EOL
Markdown:TEXT '|b|'
```

which is rendered as:

```html

|a|
|-|
|b|