feat: add 'GROUP' parameter to 'projects' cmd

[makkes]

The projects command now has a parameter GROUP:

$ gitlab projects -h
List all your projects or projects within a specific group

Usage:
  gitlab projects [GROUP] [flags]

Flags:
      --format string   Pretty-print projects using a Go template
  -h, --help            help for projects
      --member          Displays projects you are a member of
      --page int        Page of results to display (default 1)
  -q, --quiet           Only display numeric IDs

@asitemade4u would sth. like that be what you were expecting? closes #37