Move to yaml-based configuration

This commit updates the configuration to be yaml-based and updates the
configuration to read in a yaml file.

internal/config/validators/domain.go

@@ -0,0 +1,21 @@
+package validators
+
+import (
+	"regexp"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+var regexpDomain = regexp.MustCompile(`^([a-zA-Z0-9][-a-zA-Z0-9]*\.)*[a-zA-Z0-9][-a-zA-Z0-9]*$`) // TODO: Make this regexp nicer!
+
+func ValidateDomain(domain *string, dflt string) error {
+	if *domain == "" {
+		*domain = dflt
+	}
+	if !regexpDomain.MatchString(*domain) {
+		return errors.Errorf("%q is not a valid domain", *domain)
+	}
+	*domain = strings.ToLower(*domain)
+	return nil
+}