handle io::close errors

handle IO errors

use bufio::bufstream instead of io::handle

update "case let" syntax

update README.md

add ! subshell cmd

add v sqrt cmd

handle files

add ^ exponentiation cmd

fix operand order

add n 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'.

scan numbers

add IO loop

write initial interface, add Makefile

add LICENSE

initial commit