Ability to download a plugin archive

[4e554c4c]

I'm the author of a new binary (msgpack) plugin written in golang: 4e554c4c/darkman.nvim.
However, I would like to distribute the binary with the plugin, instead of requiring the user to compile it themself.
I could include the binary in the git repository with each release, but this is clumsy. Instead, I would rather create a github release (in .zip or .tar.gz format). Unfortunately this currently doesn't work very well with packer since it only supports git repos. This issue has come up before (#1107) but has not been solved.

I propose that packer support archive formats, as well as git repositories, which would allow users to distribute plugins using releases, and other non-git distribution methods.
This would add additional complexity to determining whether a file has changed (by using the if-modified-since header). However, both curl and wget support this functionality, so integrating with them would be easy.