Add documentation for default_configuration items

[michaelwood]

Add some documentation for the options that can be passed to default_configuration:

  parts = item.split()
        if len(parts) == 2 and parts[0].lower() == "skiprows" and isint(parts[1]):
            configuration["skipRows"] = max(int(parts[1]), 0)
        if len(parts) == 2 and parts[0].lower() == "headerrows" and isint(parts[1]):
            configuration["headerRows"] = max(int(parts[1]), 1)
        if len(parts) == 1 and parts[0].lower() == "ignore":
            configuration["ignore"] = True
        if len(parts) == 1 and parts[0].lower() in ("hashcomments", "hashcomment"):
            configuration["hashcomments"] = True
        if len(parts) == 2 and parts[0].lower() == "xmlroottag":
            configuration["XMLRootTag"] = parts[1]
        if len(parts) == 2 and parts[0].lower() == "rootlistpath":
            configuration["RootListPath"] = parts[1]
        if len(parts) == 2 and parts[0].lower() == "idname":
            configuration["IDName"] = parts[1]
    return configuration

[Bjwebb]

Current (limited) documentation of this is at https://flatten-tool.readthedocs.io/en/latest/unflatten/#configuration-properties-skip-and-header-rows