Add extension_preferences option

This is something for the various plugins to read settings from.

lib/Plerd.pm

@@ -223,14 +223,19 @@ has 'webmention_queue' => (
 
 has 'extensions' => (
     is => 'ro',
-    isa => 'Maybe[ArrayRef[Str]]',
+    isa => 'Maybe[ArrayRef[Str]]'
+);
+
+has 'extension_preferences' => (
+    is => 'ro',
+    isa => 'Maybe[HashRef]',
 );
 
 has 'post_triggers' => (
     is => 'ro',
     isa => 'Maybe[HashRef[Str]]',
     lazy_build => 1,
-    );
+);
 
 sub BUILD {
     my $self = shift;
@@ -823,6 +828,11 @@ the blogs's private, not-necessarily-human-readable data files.
 An arrayref of L<Str> objects, representing the plugins to load when a new Pled
 instance is created.
 
+=item extension_preferences
+
+A hashref of config options for extensions. It is up to each individual extension
+to decide how to act upon the contents therein.
+
 =item post_triggers
 
 A hashref of L<Str>,L<Str> objects. The key is used as a regex to deduce what