first and last points of stat_stat_etho should join on polar coordinates

[qgeissmann]

One of the issues of #34. Reproducible example:

library(ggetho)
metadata <- data.frame(id = sprintf("toy_experiment | %02d", 1:20),
                       condition = c("A", "B"))
dt <- toy_activity_data(metadata, 3)
pl <-  ggetho(dt, aes(y = asleep), time_wrap = behavr::hours(24), time_offset = behavr::hours(6)) + 
  stat_pop_etho() +coord_polar()   
pl

This is absolutely expected since we are drawing a line and a ribbon (and that would look terible on a non_polar coordinate system if the last and first point were joined) and not a polygon. Maybe we add an argument in stat_pop_etho such as close_path=TRUE to force drawing last line?