/lib/markdown-tables.rb:92:in `validate': No cells given (RuntimeError)

I think that a Markdown table should be rendered even if it only contains the header.

```
    if data.all? {|datum| datum.empty?}
      raise('No cells given')
    end
```

so
 
```markdown
|Header1|Header2|
|:-|:-|
```

should be formatted as

|Header1|Header2|
|:-|:-|

instead of raise a `RuntimeError`.