Make filter accept a single object

Work in progress

- [X] contexts
- [X] plurals
- [ ] make sure extraction is possible
- [ ] more tests

Example use:
```html
<div attr="{{ 'Message' | translate:'{context: \"ctx\", plural: \"\\{\\{$count\\}\\} boten\", n: \"count\"}' }}"></div>
```

Continuation of #122