Add break points for class declaration (#466)

```js
class A extends B implements C {
  method() {}
}
```

now outputs

```js
class A
  extends B
  implements C {
  method() {}
}
```

Fixes #147

tests/classes/__snapshots__/jsfmt.spec.js.snap

@@ -11,6 +11,19 @@ exports[`test binary.js 1`] = `
 "
 `;
 
+exports[`test break.js 1`] = `
+"class MyContractSelectionWidget extends React.Component<void,  MyContractSelectionWidgetPropsType, void> implements SomethingLarge {
+  method() {}
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+class MyContractSelectionWidget
+  extends React.Component<void, MyContractSelectionWidgetPropsType, void>
+  implements SomethingLarge {
+  method() {}
+}
+"
+`;
+
 exports[`test call.js 1`] = `
 "(class {})(class {});
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

tests/classes/break.js

@@ -0,0 +1,3 @@
+class MyContractSelectionWidget extends React.Component<void,  MyContractSelectionWidgetPropsType, void> implements SomethingLarge {
+  method() {}
+}