Build: allow repeated rebuilding in Jekyll Preview mode

The events and contributor plugins both monkey patched the `site`
object.  This worked fine when they were loaded once per site build, but
with Jekyll 3.0 automatic site rebuilding in preview and watch modes,
this applied the monkey patch recursively, causing the program to halt.

With this commit, the monkey patching should only occur once per run.

_plugins/contributors.rb

@@ -78,17 +78,19 @@ module Jekyll
 
     def generate(site)
       # Set site.contributors global variables for liquid/jekyll
-      class << site
-        attr_accessor :corecontributors
-        attr_accessor :sitecontributors
-        alias contrib_site_payload site_payload
-        def site_payload
-          h = contrib_site_payload
-          payload = h["site"]
-          payload["corecontributors"] = self.corecontributors
-          payload["sitecontributors"] = self.sitecontributors
-          h["site"] = payload
-          h
+      if ! site.respond_to?('corecontributors')
+        class << site
+          attr_accessor :corecontributors
+          attr_accessor :sitecontributors
+          alias contrib_site_payload site_payload
+          def site_payload
+            h = contrib_site_payload
+            payload = h["site"]
+            payload["corecontributors"] = self.corecontributors
+            payload["sitecontributors"] = self.sitecontributors
+            h["site"] = payload
+            h
+          end
         end
       end
 

_plugins/events.rb

@@ -42,15 +42,17 @@ module Jekyll
 
     def generate(site)
       # Set site.meetups and site.conferences global variables for liquid/jekyll
-      class << site
-        attr_accessor :conferences
-        alias event_site_payload site_payload
-        def site_payload
-          h = event_site_payload
-          payload = h["site"]
-          payload["conferences"] = self.conferences
-          h["site"] = payload
-          h
+      if ! site.respond_to?('conferences')
+        class << site
+          attr_accessor :meetups, :conferences
+          alias event_site_payload site_payload
+          def site_payload
+            h = event_site_payload
+            payload = h["site"]
+            payload["conferences"] = self.conferences
+            h["site"] = payload
+            h
+          end
         end
       end