Persist\tag files from GC

[dmpetrov]

We probably need a command to keep a subset of files from GC. For example, I had a previous version of my model (branch model_v1) and now I develop a new one (model_v2).

$ git checkout model_v1
$ dvc gc --tag m1 # or dvc gc --persist
$ git checkout model_v2
$ dvc gc --tag m2 # or dvc gc --persist
$ dvc gc --nuke # Remove all cache files (need to be implement) but not persistent.

Q: Do we need tags and can we utilize Git tags?

[efiop]

Added to 0.11.0 for future consideration.

Is this the same as gc --all-tags ? Is it related with #1766 ?

Closed it by mistake! My bad

[efiop]

@MrOutis #1766 mostly.