Replace zombie executable with --zombie flag

The zombie executable was symlinked, symlinks do not work properly under
jruby and rbx.

bin/mutant

@@ -1,14 +1,14 @@
 #!/usr/bin/env ruby
 
 trap('INT') do |status|
-  exit! 128+status
+  exit! 128 + status
 end
 
 require 'mutant'
 
 namespace =
-  if File.basename($0) == 'zombie'
-    $stderr.puts('Detected zombie environment...')
+  if ARGV.include?('--zombie')
+    $stderr.puts('Running mutant zombified!')
     Mutant::Zombifier.zombify
     Zombie::Mutant
   else

bin/zombie

@@ -1 +0,0 @@
-mutant

lib/mutant/cli.rb

@@ -202,6 +202,8 @@ module Mutant
         opts.separator ''
         opts.separator 'Strategies:'
 
+        opts.on('--zombie', 'Run mutant zombified')
+
         add_strategies(opts)
         add_options(opts)
       end