Add disabled runtime

lib/execjs/disabled_runtime.rb

@@ -0,0 +1,23 @@
+module ExecJS
+  class DisabledRuntime
+    def name
+      "Disabled"
+    end
+
+    def exec(source)
+      raise Error, "ExecJS disabled"
+    end
+
+    def eval(source)
+      raise Error, "ExecJS disabled"
+    end
+
+    def compile(source)
+      raise Error, "ExecJS disabled"
+    end
+
+    def available?
+      true
+    end
+  end
+end

lib/execjs/runtimes.rb

@@ -1,4 +1,5 @@
 require "execjs/module"
+require "execjs/disabled_runtime"
 require "execjs/external_runtime"
 require "execjs/johnson_runtime"
 require "execjs/mustang_runtime"
@@ -7,6 +8,8 @@
 
 module ExecJS
   module Runtimes
+    Disabled = DisabledRuntime.new
+
     RubyRacer = RubyRacerRuntime.new
 
     RubyRhino = RubyRhinoRuntime.new