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>

.eslintrc.yml

@@ -0,0 +1,34 @@
+env:
+  browser: true
+  es6: true
+  jest: true
+extends:
+  - 'eslint:recommended'
+  - 'plugin:react/recommended'
+globals:
+  Atomics: readonly
+  SharedArrayBuffer: readonly
+parser: 'babel-eslint'
+parserOptions:
+  ecmaFeatures:
+    jsx: true
+  ecmaVersion: 2018
+  sourceType: module
+plugins:
+  - react
+settings:
+  react:
+    version: 'detect'
+rules:
+  indent:
+    - error
+    - 2
+  linebreak-style:
+    - error
+    - unix
+  quotes:
+    - error
+    - single
+  semi:
+    - error
+    - always