Add exclusion list for blog request handling

[tillig]

Right now Subtext tries to handle every request coming in as a request for a blog entry. Unless it's a static file (based on a hardcoded set of extensions) or in the /images/ folder, the request is treated as a blog request. (This happens in Subtext.Framework.Web.HttpModules.BlogRequest.IsStaticFileRequest.)

If you're running your blog in the root of your site, this makes it really hard to add other non-blog pages to the site.

It would be nice to have some sort of whitelist of exclusions that you could configure such that anything in the list would specifically not be treated as a blog request.

[shankarkrupa]

Added the whitelist feature in my fork, unfortunately to the main branch :-( Awaiting action on my previous pull request so I could submit a new pull request to submit this feature.