add StringExtensions.StripHtml option

 - The StripHtml function only removes tags by default and leave html
   character codes in tact.  This change adds and option (default off)
   to the StripHtml function telling it to make a second pass which
   unencodes the html encoded characters that remain.
Merge pull request #851 from NickLarsen/v3-fixes

add StringExtensions.StripHtml option