Add ability to use CTFd config variables in challenges

[ColdHeat]

We should in theory be able to use CTFd configuration variables in things like challenge descriptions and such. I think this is would be a really useful idea.

Perhaps something like {{ ctf_name }} or {{ ctf_description }}inside of challenge descriptions.

I think we can also have a variables section within the .ctf/config section as well.

[ColdHeat]

I get the feeling that this should be supported in CTFd itself instead of via ctfcli. If this does exist I think it should be considered only with local variables that are stored in .ctf/config.