scripts: update completion scripts for bash and zsh

Files are located in: https://github.com/iterative/dvc/tree/master/scripts/completion

There are some missing flags and maybe some commands that weren't contemplated when the scripts were written.

Any other improvements are more than welcome :)

[bayethiernodiop]

gonna try to work on this in my spare time(little busy though)

[shcheklein]

@bayethiernodiop that would be great! @MrOutis do we want to reassign it then?

@bayethiernodiop , sorry I didn't see your message before!
I already worked on a patch for it, but it would be great if you can review my pull request :) !

There are some things that we are missing on zsh, tho, like completion for sub-sub-commands (e.g. dvc remote add <tab> or dvc cache dir <tab>)

#1764

Please, @bayethiernodiop , feel free to leave any comments!

[bayethiernodiop]

happy to hear that. Nice work @MrOutis i really wanted that feature.