Supporting 2.0 changes in the dvc.yaml file

The new additions are:

### Top level elements
- [ ] new `vars` keyword

###  Inside `stage`
- [ ] `desc`
- [ ] `cmd` supports list as well
- [ ] `vars`
- [ ] `foreach .. do` (they come in pair, and `do` shifts the usual stage definition)
- [x] #8

### Inside `outs`/`metrics`/`plots`
- [ ] `desc`
- [ ] `checkpoint`