Add debugging output for renderer

[corhere]

Documentation on the shape of the markdown AST produced by the blackfriday parser is rather sparse. Add a decorator for renderers which dumps the visited AST nodes to stderr, interleaved with the renderer output, to help with debugging the roff renderer.

Example

Input

Definition title
: Definition description one
: And two
: And three

MD2MAN_DEBUG=1 stderr output

+ Document: '' 
  + List: '' ListTypeDefinition|ListItemBeginningOfList
    + Item: '' ListTypeDefinition|ListTypeTerm|ListItemBeginningOfList
>> "\n.TP\n"
      + Paragraph: '' 
        + Text: 'Definition title' 
>> "Definition title"
      - Paragraph: '' 
    - Item: '' ListTypeDefinition|ListTypeTerm|ListItemBeginningOfList
>> "\n"
    + Item: '' ListTypeDefinition
      + Paragraph: '' 
        + Text: 'Definition descr...' 
>> "Definition description one"
      - Paragraph: '' 
    - Item: '' ListTypeDefinition
>> "\n"
    + Item: '' ListTypeDefinition
>> "\n"
      + Paragraph: '' 
        + Text: 'And two' 
>> "And two"
      - Paragraph: '' 
    - Item: '' ListTypeDefinition
>> "\n"
    + Item: '' ListTypeDefinition
>> "\n"
      + Paragraph: '' 
        + Text: 'And three' 
>> "And three"
      - Paragraph: '' 
    - Item: '' ListTypeDefinition
>> "\n"
  - List: '' ListTypeDefinition|ListItemBeginningOfList
- Document: '' 

[thaJeztah]

LGTM