core: Add Jim_RegisterCommand()

and wrappers Jim_RegisterCmd, Jim_RegisterSimpleCmd and Jim_RegisterSubCmd

Allows usage, help and flags when registering commands.
Allows for JIM_CMD_ISPROC, JIM_CMD_ISALIAS, JIM_CMD_ISCHANNEL
'info usage' and (later) 'info help'

Now every command doesn't need to check min/max and print usage.

Signed-off-by: Steve Bennett <steveb@workware.net.au>