Tiny typo in README.md

Minor, but feels cleaner (if we are in bash, might as well use bash-isms)

Original version required -D, -I and -O options to be listed in exactly this order. While doable, this is somewhat counter-intuitive with typical Unix option processing expectations (e.g. `ls -l -a` is expected to behave the same as `ls -a -l`). This change introduces rudimentary options processing that allows specifying them in an arbitrary order (as long as they are before any consecutive command arguments).

Better arguments parsing, plus couple tiny tweaks