Add time-varying RD/RR plots

[pzivich]

Reminder to create time-varying Risk Difference / Risk Ratio plots in the vein of Table 3 in
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4325676/ as recommended by
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4325680/

Inputs should be two pandas Series for the risks indexed by time (like lifelines fitted Kaplan Meier 1 - km.surivival_function_. Duplicated risks should be dropped, Series merged by index (time), forward filled, then plotted. Include as part of 0.1.4

Issues to consider: confidence intervals (if IP-weighted would need to be bootstrapped), scale options (like effectmeasureplot), LOWESS curve options (like functionalform).

[pzivich]

Added in v0.1.5