Add `status` command

Would be beneficial to see what git tags are not pushed right now (or possible conflicts for git tags - e.g. if you have local tag and remote but they're different).

E.g.
```
$ gto status
- Two artifacts (mymodel1, mydataset3) are added, but not committed.
Use `git add artifacts.yaml && git commit -m "your message"` to commit them.
- One new local git tag that registers a version is detected (`rf@v1.0.1`).
To push it to the repo use `gto push --tags`
```