option without 'widgetname' option to allow creating a widget object
which is used as an argument of Tcl/Tk's widget allocation commands.
* ext/tk/lib/tk/image.rb (TkImage.initialize): accept 'imagename'
option to create a image object by the given name.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@8237 b2dd03c8-39d4-4d8f-98ff-823fe69b080e