allow setting --url <custom_site_url> from commandline (issue #15, closes #143)

Sometimes you may be forced to use absolute urls. For example I had problem with FeedBurner's blog post preview page. Image links specified as relative urls were not resolved correctly. I can imagine some poor RSS readers may behave similar way.

I wanted site.url to be configurable, because I didn't want to hard-code final url there. I still want to be able to have working link when doing local web site development and testing. I have small bash script which runs jekyll -serve --auto with this parameter set for local development. My setup is more complicated, because I use several jekylls behind reverse proxy to simulate setup on http://binaryage.com which uses gh-pages from many repos together with layouts linked as git modules.

bin/jekyll

@@ -78,6 +78,10 @@ opts = OptionParser.new do |opts|
     end
   end
 
+  opts.on("--url [URL]", "Set custom site.url") do |url|
+    options['url'] = url
+  end
+
   opts.on("--version", "Display current version") do
     puts "Jekyll " + Jekyll::VERSION
     exit 0