feat: Ignore args required for strong typing correctness (#53)

* Ignore args required for strong typing correctness

In our codebase, we have this pattern:
```js
animate = (_: ?number) => {};
```

That triggers the unused-vars warning due to the argument being unused. In this instance, the argument is there to pass strong-type checks, and we have a code style guideline that uses prepending of underscore to signify  that the argument (or catch variable) is unused.

This PR adds a narrowing and widening of the no-unused-vars rule:
1.  to ignore unused arguments that begin with an underscore (a narrowing)
2. to report unused caught exceptions that don't begin with an underscore (a widening since unused caught errors was previously OFF by default)

* Add test

* derp

* Eliminate style errors in test file.

index.js

@@ -28,6 +28,7 @@ module.exports = {
     'react/no-multi-comp': [WARNING, { "ignoreStateless": true }],
     'react/prop-types': OFF,
     'react-native/no-unused-styles': ERROR,
+    'no-unused-vars': [ERROR, { 'argsIgnorePattern': '^_', 'caughtErrorsIgnorePattern': '^_' }],
     'react-native/split-platform-components': OFF,
     'react-native/no-inline-styles': WARNING,
     'react-native/no-color-literals': WARNING,

test/index.js

@@ -14,11 +14,17 @@ export default class Bool extends React.Component<Props> {
     let token = await this.current.keepOwnership();
     return this.props.updateMode(token);
   };
+
+  animate = (_: ?number) => {};
 }
 
 export function Hook() {
   React.useEffect(() => {
-    localStorage.setItem('formData', 'data');
+    try {
+      localStorage.setItem('formData', 'data');
+    } catch (_hugeObject) {
+      // handled
+    }
   });
 
   return <Bool />;