refactor(Util.escapeMarkdown): allow separate escaping and add tests (#3241)

* wip refactor

* add escapeMarkdown tests

* italics can be done with a single underscore too

* more refined

* fix test name

* unnecessary eslint ignores

* use jest

* make eslint less annoying in this test file

* more testing

* fix lib usage

* more tests and a small fix

package.json

@@ -57,6 +57,7 @@
     "@types/ws": "^6.0.1",
     "discord.js-docgen": "discordjs/docgen",
     "eslint": "^5.13.0",
+    "jest": "^24.7.1",
     "json-filter-loader": "^1.0.0",
     "terser-webpack-plugin": "^1.2.2",
     "tslint": "^5.12.1",