additional style option?

[fsolt]

Consider adding option, when plotting a single model, to do so in the style of fivethirtyeight.com:

http://mikebarnkob.dk/2016/forest-plot-showing-survival-ratios-in-ggplot2-by-emulating-fivethirtyeights-theme/

[stefan-mueller]

Sorry, I referred my PR to the wrong issue. It is actually related to #53 (allow multiple argument for alpha).