Use yihui's mime package for guess_media()

[jimhester]

It looks like you and @yihui both wrote separate implementations to guess mime types by parsing the mime.types file https://github.com/yihui/mime

As @yihui has added some R specific extensions to the lookup table it might be worth using that implementation to standardize things.

We can preserve all existing functionality with

guess_media <- function(x) { mime::guess_type(x, empty = NULL) }

[yihui]

There are a couple of packages that need to guess MIME, and I wanted to avoid copying the big list of MIME mapping info in every package, so I put together the little mime package. I have used it in the servr and markdown package, and I plan to use it in shiny as well.