Add "set" command to CLI setup menu, and add optional argument to "show".

Changes behaviour of the "show" command, it can take a param name as an optional argument and will then show only that parameter including its datatype (int8/16/32/float). Existing behaviour with no arguments is unchanged.

Example:
    show RATE_RLL_P

Also adds a new command, "set". It takes a param name and value as arguments, and sets the corresponding parameter. It does not change the datatype, and therefore does range checking on int8/int16 values.

Example:
    set RATE_RLL_P 0.123

I find it very handy since I mostly have to live without the Mission Planner.