add ics generator

Gemfile

@@ -1,2 +1,3 @@
 source "https://rubygems.org"
 gemspec
+gem 'icalendar'

_plugins/events_to_ics.rb

@@ -0,0 +1,42 @@
+require 'icalendar'
+
+module Jekyll
+  class IcsGenerator < Generator
+    safe true
+    priority :low
+
+    def generate(site)
+      events = site.collections['events'].docs
+
+      cal = Icalendar::Calendar.new
+
+      events.each do |event|
+        title = event.data['title']
+        date = event.data['event_date']
+        event = Icalendar::Event.new
+        event.dtstart = date
+        event.dtend = date
+        event.summary = title
+        cal.add_event(event)
+      end
+      site.pages << IcalPage.new(site, site.source, 'feed', "events.ics", cal)
+
+      puts "Generated .ics page from #{events.length} events"
+    end
+  end
+
+  class IcalPage < Page
+    def initialize(site, base, dir, name, calendar)
+      @site = site
+      @base = base
+      @dir  = dir
+      @name = name
+
+      self.process(name)
+      self.content = calendar.to_ical
+      self.data = {
+        'layout' => nil
+      }
+    end
+  end
+end