Fix #4, add jsx a11y linter

Note there's an error (related to #8), that isn't fixed in this commit

.eslintrc.js

@@ -6,6 +6,7 @@ module.exports = {
   "extends": [
     "eslint:recommended",
     "plugin:react/recommended",
+    "plugin:jsx-a11y/recommended",
     "plugin:mozilla/recommended"
   ],
   "parserOptions": {
@@ -13,6 +14,7 @@ module.exports = {
     "sourceType": "module"
   },
   "plugins": [
+    "jsx-a11y",
     "mozilla"
   ],
   "root": true,

package.json

@@ -10,6 +10,7 @@
     "babel-cli": "^6.26.0",
     "babel-preset-env": "^1.6.1",
     "babel-preset-react": "^6.24.1",
+    "eslint-plugin-jsx-a11y": "^6.0.3",
     "nodemon": "^1.17.3",
     "react": "^16.3.1",
     "react-dom": "^16.3.1"