[fixes #136] adding support for `processExportReassignment`

This covers these cases:

```javascript
export var a = 4;
export var b = 5 + 1;
export var c = typeof a;
export var d = a++;
export var e = ++a;

a -= 10;

--a;

export function reassign() {
  a = 10;
}

for (var i = 0; a = i, i < 10; i++) {
    console.log(i);
}
```

allowing us to control the updates on named exports for `System.register()` formatter.