Add a colour override for embed colours

- Using a new optional  value in the JSON, if set overrides the default one set by the rarity
- Added a failsafe if the override colour is invalid to default and log a warning

#349

src/contracts/SeriesMetadata.ts

@@ -12,6 +12,7 @@ export interface CardMetadata {
     type: CardRarity,
     path: string,
     subseries?: string,
+    colour?: string,
 }
 
 export interface DropResult {