[WIP] added getting the current element key

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | 
| License       | MIT

Todo.
- [ ] update documentation
- [ ] add tests
- [ ] add the same functionality for 'first' filter

This allows getting the last array key instead of only the value.
And yes I know its possible to just use [..]|keys|last but this is more clean.