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.

Author: NickLarsen

src/ServiceStack.Common/ServiceStack.Common.csproj

@@ -361,6 +361,7 @@
     <Compile Include="StringExtensions.cs">
       <SubType>Code</SubType>
     </Compile>
+    <Compile Include="StringExtensions.HtmlHelpers.cs" />
     <Compile Include="Support\ActionExecHandler.cs">
       <SubType>Code</SubType>
     </Compile>