Add frontend/bin/frontend_command.rb for development

For during development of Nullarihyon, use the command to generate Xcode build phase script.

```
$ ruby -I lib:bundle bin/frontend_command.rb check --analyzer=../build/driver/Debug/nullarihyon-core --resource-dir=../vendor/clang+llvm-3.8.0-x86_64-apple-darwin/lib/clang/3.8.0/
```

It generate command line with options converted to absolute path. Copy the output and paste it to Xcode.

frontend/bin/frontend_command.rb

@@ -0,0 +1,21 @@
+# print frontend commandline
+
+require "optparse"
+require "pathname"
+
+$analyzer_path = nil
+$resource_dir_path = nil
+
+OptionParser.new do |opt|
+  opt.on("--analyzer=PATH") {|path|
+    $analyzer_path = Pathname(path).realpath
+  }
+  opt.on("--resource-dir=PATH") {|path|
+    $resource_dir_path = Pathname(path).realpath
+  }
+end.parse!(ARGV)
+
+dir_path = Pathname(__dir__).realpath
+file_path = Pathname(__FILE__).realpath
+
+puts "/usr/bin/ruby -I#{dir_path + "lib"}:#{dir_path + "bundle"} #{file_path} xcode --analyzer=#{$analyzer_path} --resource-dir=#{$resource_dir_path}"