Tolerate delimiters wider than 1 space.

This prevents cashes on wide delimiters:

    $ ./bar_chart.py -a << EOF
    1   1
    EOF
    Traceback (most recent call last):
      File "./bar_chart.py", line 101, in <module>
        run(load_stream(sys.stdin), options)
      File "./bar_chart.py", line 48, in run
        data[kv[0]]+= int(kv[1])
    ValueError: invalid literal for int() with base 10: ''