bug#15244: eww: Enhance h1, h2 headers etc with text properties

[Jambunathan K <kjambunathan@gmail.com>]

Use case: 

Use eww to import HTML document in to Org format (or arbitrary text
markup format), the dirty way.

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How can it be achieved:

1.  The *eww* buffer can slap text properties (like :h1, :h2, :bold) etc
    on to the rendered text.

2. Emacs can offer a search and replace based on *Text Properties*.

   This way I can do, 

   - Search for a text with *italic* face

     Replace it with

        /underlying text/

   - Search for all text with h1

     Replace it with 
     
        * underlying text for h1

  - Search for all text with h2

     Replace it with 
     
        ** underlying text for h2
              
  - Search for a hyperlink

    Replace it with

        [[:href of underlying text][underlying text]]