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

@@ -78,6 +78,7 @@ module Jekyll
 
     def generate(site)
       # Set site.contributors global variables for liquid/jekyll
+      if ! site.respond_to?('corecontributors')
         class << site
           attr_accessor :corecontributors
           attr_accessor :sitecontributors
@@ -91,6 +92,7 @@ module Jekyll
             h
           end
         end
+      end
 
       # Set site.corecontributors and site.sitecontributors arrays
       site.corecontributors = {}

_plugins/events.rb

@@ -42,8 +42,9 @@ module Jekyll
 
     def generate(site)
       # Set site.meetups and site.conferences global variables for liquid/jekyll
+      if ! site.respond_to?('conferences')
         class << site
-        attr_accessor :conferences
+          attr_accessor :meetups, :conferences
           alias event_site_payload site_payload
           def site_payload
             h = event_site_payload
@@ -53,6 +54,7 @@ module Jekyll
             h
           end
         end
+      end
 
       # Set site.conferences array
       site.conferences = {}