Per-scheme proxy server

[makew0rld]

Right now there's just a a-general.proxy setting for Gemini proxies, but @LukeEmmet mentioned on IRC that it'd be nice to be able to set a different proxy server for each URL scheme.

Apparently the diohsc client does this: "Facilities to invoke external commands and use per-scheme proxies."

This should override any command settings for the scheme of the link, if they exist.

• Move a-general.proxy to a [proxies] section that works like [url-handlers]

[LukeEmmet]

Also AV-98 has a gopher proxy

Diohsc comes with a sample http proxy which wraps cURL:

Gemget has a proxy option that is agnostic to the URL scheme - this can be used for testing.