Implemented various new features.

* added support for user-supplied arguments which the user Lua scripts
  can access via the global Lua table "arg", just as in the "lua" and
  "luajit" command-line utilities. also added a test suite for this
  feature.
* added new command-line option --nginx=PATH to allow the user to
  epxlicitly specify the underlying nginx being invoked by this script.

Thanks Guanlan Dai for the patch in #1.