Added the base mechanic.

Don't tried to do something too much complicated, most application don't
need oversized intelligence on argument parsing. Compared with Arg, I just
add the following features:
- groups (it is cleaner)
- "lazy" parsing (it allows dynamic configuration to component based
  applications)
- extensibility (i can't predict all use cases)
The "lazy" parsing allows dynamic configuration to component based
applications.