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/string.go

@@ -0,0 +1,16 @@
+package validators
+
+import "github.com/pkg/errors"
+
+var errEmpty = errors.New("value is empty")
+
+func ValidateNonempty(value *string, dflt string) error {
+	if *value == "" {
+		*value = dflt
+	}
+
+	if *value == "" {
+		return errEmpty
+	}
+	return nil
+}