Add an as_of accessor

[dajmcdon]

Create a simple function to grab the asof (perhaps other metadata?)

get_edf_asof <- function(x) attr(x, "metadata")$as_of

Originally posted by @dsweber2 in cmu-delphi/epipredict#296 (comment)

[dshemetov]

Duplicate of #325. Given that the metadata is currently in flux (e.g. #523) and this only saves a few characters typing, seems low priority to me.

[brookslogan]

We do have bugs from where we use the wrong way to access the metadata and get NULL instead (I'm working on PR for that one). Might motivate us to actually do this.