Add cloudflared/Cloudflare Tunnel support

[Keyaku]

I have a server accessible via a Cloudflare Tunnel, to which I connect to with cloudflared.
Unfortunately, I can't find a way to use cloudflared in ServerBox; in Termux, I install it with pkg, and then use the SSH option ProxyCommand to make this happen.

So, I ask the following:

1. Could it be possible to add any of the available SSH options? With that, we could use ProxyCommand (as mentioned in Support for ProxyCommand in ssh #842) set to cloudflared access ssh --hostname %h.
2. Is it feasible to add a section where we can download extra executables to run locally before connection (or that allow said connection), like downloading the latest cloudflared? Or possibly use a binary installed in Termux on the same device, either through Termux:API, or accessing /data/data/com.termux/files/usr/bin?

[lollipopkit]

#949