feat(filterFilter): introduce new ":expression" syntax in pattern object

It is sometimes convenient to use an angular expression as a property key to be matched against the comparator value. This adds new syntax which can be used like this:
```javascript
{":getData().name": "M"}
```
```javascript
{":foo()": true}
```
```javascript
{":progress.current+progress.total": 100}
```

  The parsed expression function persists within the iterator closure so that performance should be fine.