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/email.go

@@ -0,0 +1,24 @@
+package validators
+
+import (
+	"regexp"
+
+	"github.com/pkg/errors"
+)
+
+var regexpEmail = regexp.MustCompile(`^([-a-zA-Z0-9]+)\@([a-zA-Z0-9][-a-zA-Z0-9]*\.)*[a-zA-Z0-9][-a-zA-Z0-9]*$`) // TODO: Make this regexp nicer!
+
+// ValidateEmail checks that s represents an email, and then returns it as is.
+func ValidateEmail(email *string, dflt string) error {
+	if *email == "" {
+		*email = dflt
+	}
+	if *email == "" { // no email provided => ok
+		return nil
+	}
+
+	if !regexpEmail.MatchString(*email) {
+		return errors.Errorf("%q is not a valid email", *email)
+	}
+	return nil
+}