Decode: unstable/Unmarshal interface (#940)

Co-authored-by: Pavlos Karakalidis <pkarakal@pkarakal.com>
Co-authored-by: Thomas Pelletier <thomas@pelletier.codes>

unstable/unmarshaler.go

@@ -0,0 +1,7 @@
+package unstable
+
+// The Unmarshaler interface may be implemented by types to customize their
+// behavior when being unmarshaled from a TOML document.
+type Unmarshaler interface {
+	UnmarshalTOML(value *Node) error
+}