Add test for unrecognized command

lib/youplot.rb

@@ -8,8 +8,10 @@ require 'youplot/command'
 module YouPlot
   class << self
     attr_accessor :run_as_executable
+
     def run_as_executable?
       @run_as_executable
     end
   end
+  @run_as_executable = false
 end