Add features of #14

- Add support for local images
- Add support for referencing Minecraft textures
Big thanks to @Hendrix-Shen

- Code cleanup

fabric/src/main/resources/fabric.mod.json

@@ -9,6 +9,10 @@
     "Motschen",
     "TeamMidnightDust"
   ],
+  "contributors": [
+    "Hendrix-Shen",
+    "Frost-ZX"
+  ],
   "contact": {
     "homepage": "https://www.midnightdust.eu/",
     "sources": "https://github.com/TeamMidnightDust/PictureSign",