Add additional search options for dir_map and dir_walk

[Moohan]

I was hoping to do a PR for this, but after having a poke around it's much more complicated than I can deal with!

I'd like to be able to do path filtering when using dir_map and dir_walk specifically, the 'missing' arguments glob, regexp and invert - I say missing as they are available in dir_ls.

Before looking at the code I assumed it would be something along the lines of dir_map = dir_ls |> purrr::map 😆 and this is essentially what I have done as a workaround.