TXT record attests ownership for the Public Suffix List

We want to place sslip.io on the Public Suffix List so we don't need to
pester Let's Encrypt for rate limit increases.

According to https://publicsuffix.org/submit/:

> owners of privately-registered domains who themselves issue subdomains
to mutually-untrusting parties may wish to be added to the PRIVATE
section of the list.

References:

- https://publicsuffix.org/
- https://github.com/publicsuffix/list/pull/2206

[Fixes #57]

integration_test.go

@@ -174,6 +174,10 @@ var _ = Describe("sslip.io-dns-server", func() {
 				"@localhost sslip.io +short +vc",
 				`\A78.46.204.247\n\z`,
 				`TypeA sslip.io. \? 78.46.204.247\n`),
+			Entry(`TXT for _psl sslip.io is a link to the pull request for putting sslip.io on the Public Suffix List`,
+				"@localhost _psl.sslip.io txt +short",
+				`\A"https://github.com/publicsuffix/list/pull/2206"\n\z`,
+				`TypeTXT _psl.sslip.io. \? \["https://github.com/publicsuffix/list/pull/2206"\]`),
 		)
 	})
 	Describe("for more complex assertions", func() {

xip/xip.go

@@ -147,6 +147,15 @@ var (
 				}, nil
 			},
 		},
+		"_psl.sslip.io.": { // avoid Let's Encrypt rate limits by joining https://publicsuffix.org
+			TXT: func(x *Xip, _ net.IP) ([]dnsmessage.TXTResource, error) {
+				x.Metrics.AnsweredTXTVersionQueries++
+				return []dnsmessage.TXTResource{
+					{TXT: []string{"https://github.com/publicsuffix/list/pull/2206"}},
+				}, nil
+			},
+		},
+
 		"metrics.status.sslip.io.": {
 			TXT: TXTMetrics,
 		},