Modernized it

* Using Yarn
* Replaced ts-lint with es-lint
* Added Prettier
* Added Husky commit hook for linting
* Added explicit return types on all methods

Author: poppa

.eslintignore

@@ -0,0 +1,6 @@
+*.d.ts
+*.js
+lib
+dist
+docs
+node_modules

.eslintrc.js

@@ -0,0 +1,33 @@
+module.exports = {
+  // Specifies the ESLint parser
+  parser: '@typescript-eslint/parser',
+  extends: [
+    // Uses the recommended rules from the @typescript-eslint/eslint-plugin
+    'plugin:@typescript-eslint/recommended',
+    // Uses eslint-config-prettier to disable ESLint rules from
+    // @typescript-eslint/eslint-plugin that would conflict with prettier
+    'prettier/@typescript-eslint',
+    'plugin:prettier/recommended',
+  ],
+  parserOptions: {
+    // Allows for the parsing of modern ECMAScript features
+    ecmaVersion: 2019,
+    // Allows for the use of imports
+    sourceType: 'module',
+  },
+  rules: {
+    // Place to specify ESLint rules. Can be used to overwrite rules specified from the extended configs
+    // "@typescript-eslint/explicit-function-return-type": "off",
+    // "eslint/prettier/prettier)"
+    'no-unused-vars': 'off',
+    '@typescript-eslint/no-unused-vars': [
+      'error',
+      {
+        vars: 'all',
+        args: 'after-used',
+        ignoreRestSiblings: true,
+        argsIgnorePattern: '^_',
+      },
+    ],
+  },
+}

.prettierignore

@@ -0,0 +1,5 @@
+*.d.ts
+lib
+dist
+docs
+node_modules

.prettierrc.js

@@ -0,0 +1,7 @@
+module.exports = {
+  semi: false,
+  trailingComma: 'all',
+  singleQuote: true,
+  trailingComma: 'es5',
+  arrowParens: 'always',
+}

jest.config.js

@@ -1,13 +1,8 @@
 module.exports = {
   transform: {
-    "^.+\\.ts$": "ts-jest"
+    '^.+\\.ts$': 'ts-jest',
   },
   roots: ['tests'],
-  testRegex: "(/__tests__/.*|(\\.|/)(test|spec))\\.(js|ts?)$",
-  moduleFileExtensions: [
-    "ts",
-    "js",
-    "json",
-    "node"
-  ],
-};
+  testRegex: '(/__tests__/.*|(\\.|/)(test|spec))\\.(js|ts?)$',
+  moduleFileExtensions: ['ts', 'js', 'json', 'node'],
+}