Pass a list of handle options to multi_download

[noamross]

multi_download() is great! Possible request - could it be possible to pass a list of lists of handle options or headers of the same length as URLs, perhaps as arguments named .handle_opts and handle_headers. This would allow one to have different handle options per URL.

My application is downloading multiple partial files where for each URL I want to get a different byte range using the range curl header option.

This can be implemented manually via the low-level multi_run() interface, but I thought it might be valuable to others in the high-level interface. (I've implemented something like it here: https://github.com/noamross/scrapetools/blob/master/R/map_curl.R),