[DOC] Manage rdoc options only in .rdoc_options file

Make `rdoc .` and `rake rdoc` consistent.

Author: nobu

.rdoc_options

@@ -2,3 +2,4 @@
 page_dir: doc
 main_page: README.md
 title: Documentation for OptionParser
+op_dir: rdoc

optparse.gemspec

@@ -24,7 +24,6 @@ Gem::Specification.new do |spec|
 
   spec.files         = Dir["{doc,lib,misc}/**/{*,.document}"] +
                        %w[README.md ChangeLog COPYING .document .rdoc_options]
-  spec.rdoc_options  = ["--main=README.md", "--op=rdoc", "--page-dir=doc"]
   spec.bindir        = "exe"
   spec.executables   = []
   spec.require_paths = ["lib"]