Add option to draw 'absolute' data set (not percentage) -

[nkammah]

aka simple …bar charts

I needed to show bar charts that were not percentages of a total. You can create such bar chart as follow:

./bin/draw_stack_bars -o foo.png "first label"=180 "another label"=50 --mode absolute

This PR also exposes additional options:

• font
• full_color
• empty_color