Add config option for primary site /blog prefix on posts permastructure

[roborourke]

Some sites need to use the /blog prefix that WP adds by default to post permalinks however we remove it currently to allow flexibility.

The blog prefix is notably desirable when using a page for posts withe slug blog.

We have a few options that would help to improve this:

• check the page for posts and it's slug, if it's blog then don't filter the permastructure option
• add a config option for whether to remove the /blog prefix, defaulting to removing it