Adding support for "-I" `$LOAD_PATH` flag

This adds support for the command-line "-I" flag to add entries to the `$LOAD_PATH`.  Both `-I <arg>` and `-I<arg>` forms are supported and the argument is always split in `File::PATH_SEPARATOR`.  Given the `while` loop parsing of argv, we get multiple-assignment support by default (yay!).

It's only remotely related, but I also added the `File::PATH_SEPARATOR` const, using `os.pathsep` for the value.