Add lubridate::period support for epi_slide_mean etc

[dsweber2]

In the process of adding step_epi_slide_mean, I noticed that given before=period("3 days") results in a window size of 1 day. either this should error or return the right value.

[brookslogan]

Was this given daily data or weekly data?

[dsweber2]

fairly sure it was daily, sorry for not including a mwe

[dshemetov]

This should be fixed by #472, where before and after types are restricted to difftimes and integers, so you'd get a sensible error.