dvc integration: summarizing already running experiment

[pared]

Scenario:

1. We run experiment, with logger turned on
2. We want to take a peek into our training
3. We run a command dvc logs show --continuous {target} (I presume dvc logs show {target} will be reserved for ad-hoc check
4. Command keeps generating new HTML until we are done with training

We need to detach summary generation from dvclive (as it is done currently).

EDIT:
probably we do not need to implement --continuous in the beginning as it is easily replaceable by simple watch command.

[dmpetrov]

I'd expect that the Summary&HTML files regeneration will happen automatically without any additional command. We can consider a suppression flag if needed.

[pared]

@dmpetrov ok, do we want to keep dvc logs command functionality? I imagine there can be use cases for that - eg when someone just wants to see comparison of two different training runs, on a fresh repo clone.

[pared]

We introduced command for producing run summary as in #16, in iterative/dvc#4902