Added ability to ingest aggregated data

Added this in bar_chart, and documentation. (If you like this, should be easy to add to histogram too, let me know). See the following test run that illustrates it:

        *** Contents of a:
        1
        2
        3
        3
        4
        5
        5
        6
        6
        *** Contents of b:
        1 1
        2 1
        3 11
        3 5
        4 5
        5 7
        5 8
        6 5
        6 1
        *** cat a | ./bar_chart.py:
        # each * represents a count of 1
        1 [     1] *
        2 [     1] *
        3 [     2] **
        4 [     1] *
        5 [     2] **
        6 [     2] **
        *** cat b | ./bar_chart.py -a:
        # each * represents a count of 1
        1 [     1] *
        2 [     1] *
        3 [    16] ****************
        4 [     5] *****
        5 [    15] ***************
        6 [     6] ******
        *** cat b | ./bar_chart.py -a -r:
        # each * represents a count of 1
        6 [     6] ******
        5 [    15] ***************
        4 [     5] *****
        3 [    16] ****************
        2 [     1] *
        1 [     1] *