Heading detection should not be applied to the output of paragraph reflowing

[jpduffield]

If reflowing a paragraph results in a final line containing only one word, then that word is incorrectly turned into a level 3 heading.
E.g. the following comment paragraph:

  // Package foo provides an example of how text width handling in gomarkdown 
  // can cause the last word of a paragraph to be placed on a line by itself.
  // It looks like processing may cause that word to have heading formatting
  // applied incorrectly   

results in the following markdown:

Package foo provides an example of how text width handling in gomarkdown can
cause the last word of a paragraph to be placed on a line by itself. It looks
like processing may cause that word to have heading formatting applied
### incorrectly