plotHeatmapMedianWeeks weekdays name issues

[shostiou]

Hi, thanks for this really helpful package.
My Debian 10 OS is configured to use french as a local language.
While making calls to plotHeatmapMedianWeeks weekdays aren't properly converted to English.
As a consequence, y axis of the plot is not segmented correctly.

The issue can be fixed by correcting the value of the "locale" argument passed to the lubridate::wday function
used to convert weekdays to English :
locale = "en_US.utf8" instead of locale = "en_US".