Removes code duplication (code climate)

No gain but it's "better": https://codeclimate.com/github/Mogztter/asciidoctor/compare/code-climate
The idea is to break the method into smaller pieces.
I can "map" the table alignments into an Hash to stick to "functional" method:
```ruby
  def self.parse_col_row_spec(colspec, rowspec)
    spec = {}
    if !colspec.nil_or_empty? && Table::ALIGNMENTS[:h].has_key?(colspec)
      spec['halign'] = Table::ALIGNMENTS[:h][colspec]
    end
    if !rowspec.nil_or_empty? && Table::ALIGNMENTS[:v].has_key?(rowspec)
      spec['valign'] = Table::ALIGNMENTS[:v][rowspec]
    end
    spec
  end
```

Then merge the two Hash:

```ruby
spec.merge(parse_col_row_spec(colspec, rowspec))
```

Prettier ? What do you suggest ?