Allows to view commit history in extended way

Instead of git fetch && git rebase.

I've changed it to use `view` instead of `vim -R` to make it hopefully
more editor-agnostic.

In detail:
gup - now completes with remotes like git fetch would
gdv - got git diff completions
gc/gca - now zsh will suggest files in staging area
gba - suggestions as for git branch

Reorganizing plugins so that each plugin has it's own directory now so that any plugin-specific functions can be bundled within there.