Added POKE but it's not useful yet.

POKE will only be useful when the memory map is stabilized.
I suppose in this early release you could POKE into bits of
program memory provided by the VERIFY dump, or you could
sit there with a symbol table to find out where to poke.
when I implement PEEK this might be more useful.
Moved load/save/verify into a separate source file.

Preparing to add a stub to LOAD to load a demo program that is
compiled into the binary.