Add support for defining top attributes for a string. 

This change allows to make further changes to the parsed string. Some of them will be overridden by other properties such as ``paragraphFont`` or ``boldFontName``. Usage example:

```
  NSAttributedStringMarkdownParser * parser =[[NSAttributedStringMarkdownParser alloc] init];

  NSMutableParagraphStyle * para = [[NSMutableParagraphStyle alloc] init];
  para.minimumLineHeight = 50;
  parser.topAttributes = @{NSForegroundColorAttributeName : UIColor.greenColor,
                           NSParagraphStyleAttributeName : para,
                           NSStrikethroughStyleAttributeName : @1};
```