Add filter for favicon file path

Genesis child themes (and probably others) have their favicon.ico files at {child-theme}/images/favicon.ico since this is where Genesis expects them. This doesn't match with where this plugin is looking for them.

This change refactors out the multiple occurrences of '/favicon.ico' into a new method, and adds a filter so it can be changed as needed. No extra information is passed to the filter other than the default location, but filtering functions can grab blog IDs and whatever other information it needs to contextually return specific values.