Removes code duplication (code climate)

No gain but it's "better": https://codeclimate.com/github/Mogztter/asciidoctor/compare/code-climate
Something like this:

```ruby
colspec, rowspec = m[3].split '.'
spec['halign'] = Table::ALIGNMENTS[:h][colspec]
spec['valign'] = Table::ALIGNMENTS[:v][rowspec]
...
spec.delete_if { |_, v| v.nil_or_empty? }
```

Note: It’s not necessary to check if a hash contains a key to get its value — when the key doesn’t exist (or is `nil`), it just returns `nil`.