Allow to differentiate between certain and likely seats

[filipvanlaenen]

E.g.:

• Certain seats: lower bound of 95% confidence interval, filled
• Likely seats: between lower bound of 95% confidence interval and median, hatched
• Unlikely seats: above median, stroke

Specified from the command line as 5:7:8 instead of a simple number. 5:7 indicates no unlikely seats

Requires a legend, and perhaps the possibility to set a custom text in the legend