Add auto spacing for charts

Hello,
I just started using timewarrior and noticed that it would be great if there would be a setting to set the spacing of a chart to be chosen automatically to filling the whole screen width.
So if someone would to specify `rc.reports.week.spacing=auto` the correct spacing for the current terminal would be calculated and used so that the chart would fill the entire width of the terminal.

Thank you all very much for this great project!