Align behaviour between bundle exec rdoc and rake rdoc

By using `.rdoc_options` to specify the RDoc options, we can make most
of the rake task options the in-repo defaults.

This allows us to use the same options for both `bundle exec rdoc` and
`rake rdoc` and get a consistent experience.

.document

@@ -1,7 +1,7 @@
 History.txt
 LICENSE.txt
 README.txt
-RI.txt
-RI.md
+*.md
+*.rdoc
 lib
 doc

.rdoc_options

@@ -0,0 +1,8 @@
+exclude:
+  - lib/rdoc/rd/block_parser.ry
+  - lib/rdoc/rd/inline_parser.ry
+  - lib/rdoc/markdown.kpeg
+  - lib/rdoc/markdown/literals.kpeg
+op_dir: _site # for GH pages and should match the config of RDoc task in Rakefile
+title: rdoc Documentation
+main_page: README.rdoc

Rakefile

@@ -21,10 +21,8 @@ task :default => :test
 
 task rdoc: :generate
 RDoc::Task.new do |doc|
-  doc.main = 'README.rdoc'
-  doc.title = "rdoc #{RDoc::VERSION} Documentation"
-  doc.rdoc_dir = '_site' # for github pages
-  doc.rdoc_files = FileList.new %w[lib/**/*.rb *.rdoc *.md doc/rdoc/markup_reference.rb] - PARSER_FILES
+  # RDoc task defaults to /html and overrides the op_dir option in .rdoc_options
+  doc.rdoc_dir = "_site" # for github pages
 end
 
 task "coverage" do