saveStart( fpath )
    Starts saving commands to a specified file path.

    Parameters
    ----------
    fpath: string
        File path.

    Returns
    -------
    id: number
        Record identifier.

    Examples
    --------
    > var id = saveStart( './path/to/file.txt' );
    > 1 + 1
    2
    > 'foo'
    'foo'
    > saveStop( id );

    See Also
    --------
    save, saveStop