Add support for ISO 8601 date format

[arteymix]

This is related to #18

So, ISO 8601 is not part of base R and attempting to parse it using strptime will not deal properly with the timezone. Ugh.

I learned that is.integer does not return TRUE if provided with an integer.

I think this would be missing a library(lubridate) call somewhere to import the relevant functions.

Also, lubridate is part of Tidyverse. Is it something that is desirable?

Alternatives I considered:

• strptime from base R, but does not deal nicely with TZ
• parsedate
• parse_datetime from readr, though that is a big dependency