erb: lineno and location setters

* lib/erb.rb (ERB#lineno): accessor for line number to eval.
* lib/erb.rb (ERB#location=): setter of file name and line number.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48785 b2dd03c8-39d4-4d8f-98ff-823fe69b080e