Migrate over to certmagic from using autocert (#190)

* Use certmagic for challenge validation

* WIP

* Get the correct key

* Override preflight check logic

* Fix logging for imported packages and tidy config.cfg

* Fix test and add docstrings

* Update README

challengeprovider.go

@@ -0,0 +1,30 @@
+package main
+
+import "github.com/go-acme/lego/challenge/dns01"
+
+// ChallengeProvider implements go-acme/lego Provider interface which is used for ACME DNS challenge handling
+type ChallengeProvider struct {
+	servers []*DNSServer
+}
+
+// NewChallengeProvider creates a new instance of ChallengeProvider
+func NewChallengeProvider(servers []*DNSServer) ChallengeProvider {
+	return ChallengeProvider{servers: servers}
+}
+
+// Present is used for making the ACME DNS challenge token available for DNS
+func (c *ChallengeProvider) Present(_, _, keyAuth string) error {
+	_, token := dns01.GetRecord("whatever", keyAuth)
+	for _, s := range c.servers {
+		s.PersonalKeyAuth = token
+	}
+	return nil
+}
+
+// CleanUp is called after the run to remove the ACME DNS challenge tokens from DNS records
+func (c *ChallengeProvider) CleanUp(_, _, _ string) error {
+	for _, s := range c.servers {
+		s.PersonalKeyAuth = ""
+	}
+	return nil
+}