Only remove /blog or blog/ at the start of the URL

HI,

I have made a change to your plugin to allow for categories which may have be named 'blog' and have a sub-category under them, as this would create a rather confusing  link for the sub-categories.

This will now only replace 'blog/' or '/blog/' at the start of the link.

I have also changed the check for multisite and main site to use the built in function, as this would previously not work if the main site had been changed form the default site.

Thanks for the plugin, and I hope these changes help!