Added wkday feature to create_sunset_cal.R

Since I live in two localities, I wanted one calendar for weekday sundowns in one location and another calendar for weekend sundowns in another location. So I added a new variable **wkday** that lets me choose the weekdays for which I need the calendar.

**I have not been able** to compile the changes to the documentary and the package (I am quite new to R, and still failed to do it after reading your blogpost). So while the new option does what it should, you should probably put together the package and documentation again. 

I added to the documentation comments in the file though, so the new feature will be documented.