Include default changefreq

[thany]

change_frequency_name is a nice idea, but when implementing this plugin after a site has been built, it requires going through all pages and putting a changefreq in them.

This is not cool if the changefreq is going to be the same for most pages. Therefore, I propose including a setting like "default_change_frequency" that controls, when set, the changefreq that gets added to pages where the change_frequency_name setting has not been set.