Summary:
This fixes the indentation for expression arrow functions
that are continued on multiple lines.
This does not affect the `FunctionBody` variant of arrow functions,
which were already correct due to the opening brace.

Resolves #332.

Test Plan:
Type
```javascript
const myNiftyArrowFunction = (foo, bar) =>
    foo + bar;
myNiftyArrowFunction(1, 2);
```
and note that it's indented correctly as you type.
Then run `gg=G` and make sure that it stays indented correctly.

Note also that the indentation of
```javascript
const myFunctionBodiedArrowFunction = (foo, bar) => {
    doAThing();
    return foo + bar;
};
myFunctionBodiedArrowFunction(1, 2);
```
is still correct.