Semantic Analysis Fine Tuning

- Finish URL substitution.

- Use HREF or SRC as display/alt text, if no valid default content
  is provided for hyperlinks and images.

- Correct infinite loop in pruneDL().

- Keep <hr> in the tree.

- Simplify inline pruning: If an inline element contains
  only blank text, or elements containing only blank text,
  it'll be removed.

  E.g: Red''{Lots of space}''Beer -> <p>RedBeer</p>