Add a help feature to bspc command, Fix #776

[badele]

Add help for command line

Example for:

> bspc 

bspc usage:
  node
  desktop
  monitor
  query
  wm
  rule
  subscribe
  config
  quit

> bspc desktop                                                                                                                                                                            

desktop usage:
  -f, --focus [DESKTOP_SEL]             Focus the selected or given desktop
  -a, --activate [DESKTOP_SEL]          Activate the selected or given desktop
  -m, --to-monitor MONITOR_SEL          Send the selected desktop to the given monitor
  -s, --swap DESKTOP_SEL                Swap the selected desktop with the given desktop
  -b, --bubble CYCLE_DIR                Bubble the selected desktop in the given direction
  -l, --layout CYCLE_DIR|monocle|tiled  Set or cycle the layout of the selected desktop
  -n, --rename <new_name>               Rename the selected desktop
  -r, --remove                          Remove the selected desktop
  -h,--help                             Help