Feature: Client window action

- Add support for client window actions. (#2)

- Fix access to uninitialized memory in config_parse().

- Fix a bug that the first command sent to an existing pipe is ignored.

- Add --config to specify path to configuration file. Move commandline
  argument parsing to a separate function parse_args(), and split the
  process to two passes.

- Move "dpy", "root", "screen", etc. to session_t.

- Code cleanup.

- Known issue: Mini window handling looks horribly broken.