Add SET Function

[gavbarnett]

Implement SET function similar to CMD on windows.
This allows long strings or values to be reduced to simple variables.
Add SET and an associated memory space (array)
Add functionality to ECHO to decode variables

SET a=6
SET b=3
SET c=cat
ECHO %a%
6
ECHO %b%
3
ECHO a
a
Echo Thats a nice %c% that lives at %b% queens street.
Thats a nice cat that lives at 3 queens street.
SET  //prints all variables
a=6
b=3
c=cat

Later consider extending with math & array functions