Add array of supported XEPs

[chrisballinger]

Use XEPper tool to get server features: https://davidchiles.github.io/XEPper/

@davidchiles Can we modify XEPper to also produce JSON output of supported XEPs?

This will be useful in the future for determining if a server has XEP-0357 / push support.

{
"domain": "dukgo.com",
"extensions": ["XEP-0198", "XEP-0077", "RFC-6121"]
}

[chrisballinger]

We now have a much better way of doing this client-side

[dllud]

Nowadays it would be better to use the XMPP Compliance Tester by @iNPUTmice as it scans many more XEPs. You wouldn't even need to run it, you could just parse the available reports.

[chrisballinger]

Agreed! We should write a script that pulls in the results from the reports because running the compliance tester is a pain. Right now the only critical XEPs are 0357 and 0363, but that will probably change over time.