bin/osh is compatible with POSIX shell, bash, and other shells.

Usage: osh FLAG* SCRIPT ARG*
       osh FLAG* -c COMMAND ARG*
       osh FLAG*

The command line accepted by `bin/osh` is compatible with /bin/sh and bash.

    osh -c 'echo hi'
    osh myscript.sh
    echo 'echo hi' | osh

It also has a few enhancements:

    osh -n -c 'hello'                    # pretty-print the AST
    osh --ast-format text -n -c 'hello'  # print it full

osh accepts POSIX sh flags, with these additions:

    -n             parse the program but don't execute it.  Print the AST.
    --ast-format   what format the AST should be in
