(refactor): get tsdx lint working with standard style

- replace standard with eslint and all eslint-config-standard etc
- hopefully tsdx lint should be more compatible with editors, tools,
  TS, etc than standard, because it gave lots of issues
  - lots of things, esp. integrations, are pretty unmaintained in
    standard's ecosystem :(
  - add react-app and prettier from tsdx lint --write-file so editors
    etc will be able to have the same config as TSDX internally
  - have to turn off prettier as double quotes and semicolons conflict
    with standard

- fix lint errors in the example code

(deps): add eslint-config-standard and all its peerDeps
(deps): add eslint-plugin-react as tsdx lint will error without it

.eslintrc.js

@@ -0,0 +1,10 @@
+module.exports = {
+  ignorePatterns: ['dist/'],
+  extends: [
+    'react-app',
+    'standard'
+  ],
+  rules: {
+    'prettier/prettier': 'off' // override tsdx lint
+  }
+}

example/app.js

@@ -15,8 +15,9 @@ class App extends Component {
   }
 
   trim = () => {
-    this.setState({ trimmedDataURL: this.sigPad.getTrimmedCanvas()
-      .toDataURL('image/png') })
+    this.setState({
+      trimmedDataURL: this.sigPad.getTrimmedCanvas().toDataURL('image/png')
+    })
   }
 
   render () {
@@ -35,7 +36,7 @@ class App extends Component {
         </button>
       </div>
       {trimmedDataURL
-        ? <img className={styles.sigImage}
+        ? <img className={styles.sigImage} alt='signature'
           src={trimmedDataURL} />
         : null}
     </div>