[Request] cURL custom certificate

[TurtleP]

This has a bit of discussion between AuahDark and myself, but I may as well open the issue for tracking purposes and get more thoughts.

Similarly with #16, updating lua-https only for this change would be a pain, as I would need to generate a patch file and apply it during my CMake configuration.

There should be a way to optionally provide a custom certificate that cURL can use. The discussions with AuahDark have shown two possibilities:

1. Add a compile flag to CMake defining the path to the certificate.
2. Expose a lua method to set the certificate path, which could be useful for games made with LÖVE.