handle io::close errors
use bufio::bufstream instead of io::handle
update "case let" syntax
add ! subshell cmd
add ^ exponentiation cmd
add negative numbers and stack control commands
scan decimal points, ignore blanks, add more ops

Full floating point numbers like 3.14 are scanable.
All blank characters like spaces, tabs and newlines are ignored.
Added multiplication '*', division '/', modulus '%', quit 'q'.
write initial interface, add Makefile