Add mechanism to easily get server side js string for debug

[catmando]

when debugging server side rendering if you encounter errors you need to see the files generated.

module MiniRacer
  class Context
    alias original_eval eval

    def eval(str, options = nil)
      original_eval str, options
    rescue Exception => e
      raise e
    ensure
      FileUtils.mkdir_p('./tmp') unless File.directory?('./tmp')
      File.write('./tmp/prerendered-source.js', str)
    end
  end
end

Something like the above mini racer patch with the possibility of a config switch to set the file path. empty path means don't generate it.