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/slug.go

@@ -0,0 +1,24 @@
+package validators
+
+import (
+	"regexp"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+var regexpSlug = regexp.MustCompile(`^[a-zA-Z0-9][-a-zA-Z0-9]*$`) // TODO: Make this regexp nicer!
+
+var ErrInvalidSlug = errors.New("invalid slug")
+
+// ValidateSlug validates a slug and normalizes it.
+func ValidateSlug(s *string, dflt string) error {
+	if *s == "" {
+		*s = dflt
+	}
+	*s = strings.ToLower(*s)
+	if !regexpSlug.MatchString(*s) {
+		return ErrInvalidSlug
+	}
+	return nil
+}