Add command line handlers

Each handler is registered with a command name, and then called with arguments
parsed. There is a built-in help command.

General help:
  ./eclipse help

Command help:
  ./eclipse help base

Command usage:
  ./eclipse base --arg1 val1

It aims to replace the command line handling in the RCP.

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>