Consider supporting [this|next|last] ... syntax (i.e., multiple modifiers) #23

It was decided that #21 isn't useful enough as compared to the potential confusion it might cause due to the complication of viewcal's syntax. But something like:

viewcal next next today

(show the events for two days from now),

viewcal last last week

(show the events for the week before last),

and so on, while more verbose, does basically the same thing and fits more naturally with the current syntax.

This seems like it should be easily implementable by using the nargs argument to ArgumentParser's add_argument() method.