chore: fix ESLint errors

Now ESLint is configured so it can run across the entire codebase
without errors.

Author: silvenon

.eslintignore

@@ -1,3 +1,4 @@
 **/node_modules
 www/.cache/
 www/public/
+lib

.eslintrc.yml

@@ -8,3 +8,11 @@ env:
   browser: true
 plugins:
   - jsx-a11y
+
+overrides:
+  - files: www/**/*
+    env:
+      es6: true
+  - files: stories/**/*
+    rules:
+      no-console: off

rollup.config.js

@@ -1,15 +1,15 @@
-import nodeResolve from "rollup-plugin-node-resolve";
-import babel from "rollup-plugin-babel";
-import commonjs from "rollup-plugin-commonjs";
-import replace from "rollup-plugin-replace";
-import { sizeSnapshot } from "rollup-plugin-size-snapshot";
-import { terser } from "rollup-plugin-terser";
+import nodeResolve from 'rollup-plugin-node-resolve';
+import babel from 'rollup-plugin-babel';
+import commonjs from 'rollup-plugin-commonjs';
+import replace from 'rollup-plugin-replace';
+import { sizeSnapshot } from 'rollup-plugin-size-snapshot';
+import { terser } from 'rollup-plugin-terser';
 
-const input = "./src/umd.js";
-const name = "ReactTransitionGroup";
+const input = './src/umd.js';
+const name = 'ReactTransitionGroup';
 const globals = {
-  react: "React",
-  "react-dom": "ReactDOM"
+  react: 'React',
+  'react-dom': 'ReactDOM'
 };
 
 const babelOptions = {
@@ -20,16 +20,16 @@ const babelOptions = {
 const commonjsOptions = {
   include: /node_modules/,
   namedExports: {
-    "prop-types": ["object", "oneOfType", "element", "bool", "func"]
+    'prop-types': ['object', 'oneOfType', 'element', 'bool', 'func']
   }
 };
 
 export default [
   {
     input,
     output: {
-      file: "./lib/dist/react-transition-group.js",
-      format: "umd",
+      file: './lib/dist/react-transition-group.js',
+      format: 'umd',
       name,
       globals
     },
@@ -38,16 +38,16 @@ export default [
       nodeResolve(),
       babel(babelOptions),
       commonjs(commonjsOptions),
-      replace({ "process.env.NODE_ENV": JSON.stringify("development") }),
+      replace({ 'process.env.NODE_ENV': JSON.stringify('development') }),
       sizeSnapshot()
     ]
   },
 
   {
     input,
     output: {
-      file: "./lib/dist/react-transition-group.min.js",
-      format: "umd",
+      file: './lib/dist/react-transition-group.min.js',
+      format: 'umd',
       name,
       globals
     },
@@ -56,7 +56,7 @@ export default [
       nodeResolve(),
       babel(babelOptions),
       commonjs(commonjsOptions),
-      replace({ "process.env.NODE_ENV": JSON.stringify("production") }),
+      replace({ 'process.env.NODE_ENV': JSON.stringify('production') }),
       sizeSnapshot(),
       terser()
     ]