Convert an input description into command line argument.

def nf_input_to_cl(inp):
    """Convert an input description into command line argument.
    """
    sep = " " if inp.get("separate") else ""
    val = "'%s'" % inp.get("default") if inp.get("default") else "$%s" % inp["name"]
    return "%s%s%s" % (inp["prefix"], sep, val)