Retrieve contents of the description string from a Feather file

[dmbates]

See the slack discussion https://julialang.slack.com/archives/C674VR0HH/p1582304415448000.

It would be handy to have, say, Feather.description to return the description string. Would it be worthwhile creating a Feather.File type, analogous to CSV.File, that would just go through the loaddata step (with validation) and perhaps create the CTable from the Vector{UInt8} of the memory-mapped file.