fix: undefined method excerpt_separator (#66)

In Jekyll, only the Jekyll::Document has excerpt_seperator, to fix
this issue, we should filter the objects which aren't instances of
Jekyll::Document.

Author: jeffreytse

lib/jekyll-spaceship/cores/manager.rb

@@ -68,7 +68,8 @@ def self.dispatch(page, container, event)
       end
       # update page excerpt
       @@_processors.each do |processor|
-        next if not processor.handled
+        next unless processor.handled
+        next unless page.is_a? Jekyll::Document
         break page.data['excerpt'] = Jekyll::Excerpt.new(page)
       end
       # call on_handled