tools/btgatt-client: Added the "initialize" command.

Added the "initialize" command which triggers GATT service, characteristic, and
descriptor discovery procedures and populates the bt_gatt_client instance. Also
added some (shitty) coloring for verbose output.
tools/btgatt-client: Add "services" command.

Added the "services" command, which enumerates all discovered services,
characteristics, and descriptors.
tools/btgatt: Add command argument support.

Added support for command arguments and added the -u and -a flags to the
"services" command to filter services by UUID or by start handle.