Feature request: conf files/directory supporting.

Hi,

I'm very glad to know that dnsproxy has implemented [this feature](https://github.com/AdguardTeam/dnsproxy#specifying-upstreams-for-domains) currently. But the problem is that I've so many domains which I want to write in this form, for this purpose, the dnsmasq's scheme, i.e., supporting for using/selecting conf files from specific directory is very convenient. Could you please add this feature into dnsproxy?

Regards,
HY