Add rdoc options

Specify the main page and the page directory.  Also in Rakefile,
extract and use the same options from the gemspec file.

Author: nobu

Rakefile

@@ -16,5 +16,5 @@ end
 task :default => :test
 
 task :rdoc do
-  sh("rdoc", "--op", "rdoc")
+  sh("rdoc", *Bundler::GemHelper.instance.gemspec.rdoc_options, ".")
 end

optparse.gemspec

@@ -23,6 +23,7 @@ Gem::Specification.new do |spec|
   spec.metadata["source_code_uri"] = spec.homepage
 
   spec.files         = Dir["{doc,lib,misc}/**/*"] + %w[README.md ChangeLog COPYING]
+  spec.rdoc_options  = ["--main=README.md", "--op=rdoc", "--page-dir=doc"]
   spec.bindir        = "exe"
   spec.executables   = []
   spec.require_paths = ["lib"]