+ DNS: add "ipset" configuration setting

Close #1191

Squashed commit of the following:

commit ba14b53f9e3d98ad8127aa3af1def0da4269e8c4
Merge: 362f4c44 6b614295
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Wed Sep 2 14:03:19 2020 +0300

    Merge remote-tracking branch 'origin/master' into 1191-ipset

commit 362f4c44915cb8946db2e80f9a3f5afd74fe5de1
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Wed Sep 2 12:50:56 2020 +0300

    minor

commit 28e12459166fe3d13fb0dbe59ac11b7d86adb9b4
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Wed Sep 2 12:43:25 2020 +0300

    minor

commit bdbd7324501f6111bea1e91eda7d730c7ea57b11
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Tue Sep 1 18:40:04 2020 +0300

    move code, ipset-v6

commit 77f4d943e74b70b5bc5aea279875ab1e2fab2192
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Tue Sep 1 15:53:27 2020 +0300

    comment

commit 16401325bbefeba08e447257b12a8424b78c9475
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Aug 31 17:43:23 2020 +0300

    minor

commit c8410e9a519b87911bc50f504e8b4aaf8dce6e02
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Aug 31 15:30:52 2020 +0300

    + DNS: add "ipset" configuration setting

dnsforward/config.go

@@ -82,6 +82,11 @@ type FilteringConfig struct {
 	EnableDNSSEC           bool     `yaml:"enable_dnssec"`      // Set DNSSEC flag in outcoming DNS request
 	EnableEDNSClientSubnet bool     `yaml:"edns_client_subnet"` // Enable EDNS Client Subnet option
 	MaxGoroutines          uint32   `yaml:"max_goroutines"`     // Max. number of parallel goroutines for processing incoming requests
+
+	// IPSET configuration - add IP addresses of the specified domain names to an ipset list
+	// Syntax:
+	// "DOMAIN[,DOMAIN].../IPSET_NAME"
+	IPSETList []string `yaml:"ipset"`
 }
 
 // TLSConfig is the TLS configuration for HTTPS, DNS-over-HTTPS, and DNS-over-TLS