Add ESLint to maintain code style. Add pre-commit hook to lint prior.

ESLint is used with the standard react plugin. It detects all kinds of
issues ranging from misspells, indentation, variable-naming, etc.
A pre-commit hook is added to git. Prior commiting, ESlint will run
to validate that everything is OK and the user will have the option
to fix it.

Signed-off-by: Sudhanshu Gautam <me@sudhanshug.com>

package.json

@@ -24,7 +24,8 @@
     "test": "react-scripts test",
     "eject": "react-scripts eject",
     "predeploy": "yarn run build",
-    "deploy": "gh-pages -d build"
+    "deploy": "gh-pages -d build",
+    "lint": "eslint src/ --ext .js --fix"
   },
   "eslintConfig": {
     "extends": "react-app"
@@ -40,5 +41,12 @@
       "last 1 firefox version",
       "last 1 safari version"
     ]
+  },
+  "devDependencies": {
+    "babel-eslint": "^10.0.2",
+    "eslint": "^6.1.0",
+    "eslint-plugin-react": "^7.14.2",
+    "prop-types": "latest",
+    "typescript": "^3.5.3"
   }
 }