erb: set variables from the command line

* bin/erb (ARGV.switch, ERB::Main#run): allow variables to be set
  from the command line.  [ruby-core:65772] [Feature #10395]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48786 b2dd03c8-39d4-4d8f-98ff-823fe69b080e