Add feature to descend into subdir of NEWPWD

To descend into a subdir, one would have to do
    cd `bd backdir`/subdir
Instead, the -d option will achieve the same result
    . bd -s backdir -d subdir

Required adding a new option -d so switched to using getopts
instead of having to parse arguments using nested if's
To simplify things, made -i into another seperate flag instead of the -si opt
added -h flag, to display usage_error
added -v flag, for verbose output