message: making mysql optinal dependency
diff: {
  "name": "pigallery2",
  - "version": "1.1.0",
  + "version": "1.2.0",
  "description": "This is a photo gallery optimised for running low resource servers (especially on raspberry pi)",
  "author": "Patrik J. Braun",
  "homepage": "https://github.com/bpatrik/PiGallery2",
  "express": "4.16.3",
  "jimp": "0.2.28",
  "locale": "0.1.0",
  - "mysql": "2.15.0",
  "reflect-metadata": "0.1.12",
  "sqlite3": "4.0.0",
  "ts-exif-parser": "0.1.23",
  "zone.js": "0.8.26"
  },
  "optionalDependencies": {
  + "mysql": "2.15.0",
  "bcrypt": "2.0.1",
  "gm": "1.23.1",
  "sharp": "0.20.2"
  