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 when applying favicons to the admin bar.

This change refactors out the multiple occurrences of `'/favicon.ico'` into a new method, and applies 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.

For a site that only used Genesis child themes, this can be as simple as:

~~~php
add_filter( 'multisite_enhancements_favicon_path', 'bcuk_favicon_path' );
/**
 * Tell Multisite Enhancements plugin where to find favicon in Genesis child themes.
 *
 * @return string File path to favicon file.
 */
function bcuk_favicon_path() {
	return '/images/favicon.ico';
}
~~~
in a MU or network activated plugin.

Note that the filter can also be used to return a different file type or completely different file name e.g. `/images/multisite-enhancements-admin-bar-logo.png`.