Support HTML DEL and INS elements.

[Zegnat]

Noticed this when I was using the Pismo powered ‘entry text extraction’ on Feedbin.

>> Pismo['http://hsivonen.iki.fi/accept-charset/'].lede
=> "Accept-Charset Is No More. Now that Firefox 10 has been released, the Accept-Charset HTTP header. During the Firefox 4 development cycle, I noticed that IE and Safari were not sending the Accept-Charset HTTP header in their HTTP requests. This meant that the Web had to work even without browser sending that header." 

The first sentence given by Pismo:

Now that Firefox 10 has been released, the Accept-Charset HTTP header.

Comes from the following HTML:

<p>Now that Firefox 10 has been released, <del>none of the major browsers send</del> <ins>only Chrome sends</ins> the <code>Accept-Charset</code> HTTP header.</p>

If anything I would have expected Pismo to drop the DEL elements but keep the INS elements like so:

Now that Firefox 10 has been released, only Chrome sends the Accept-Charset HTTP header.

Even the html_body does not return these tags. This means possible important parts of a document can go missing. See this return, edited to only show the first paragraph:

>> Pismo['http://hsivonen.iki.fi/accept-charset/'].html_body
 => "Accept-Charset Is No More<p>Now that Firefox 10 has been released,   the <code>Accept-Charset</code> HTTP header.</p>\n\n"

Please support the DEL and INS elements by:

1. Drop the DEL and its content from lede and body but keep the content of INS in both.
2. Keep the DEL and INS elements and their content in html_body.