Flags before command don't work
```
» ../../nimble --depsOnly install                                                      1 ↵
     Error: /Users/dom/projects/nimble/src/nimble.nim(1187) nimble
        ... /Users/dom/projects/nimble/src/nimblepkg/options.nim(421) parseCmdLine
        ... /Users/dom/projects/nimble/src/nimblepkg/options.nim(400) handleUnknownFlags
        ... Unknown option: --depsOnly
```

This is a regression