Add Profile field support to HTTP ContentType (resolves #127)

[puzzler995]

Modified ActivityPubOptions and ActivityPubClient to instead have a ContentType record struct, so it can handle both MediaType and Profile.

Feel free to let me know if there's anything I need to change. I've tested it as much as I can, but if you know of any softwares that actually follow the spec on this that I can test against, I will for sure.