`nimble develop -g` doesn't work
```
options.nim(661)         handleUnknownFlags

    Error:  Unknown option: -g
```

https://github.com/nim-lang/nimble/blob/a0a12f475905028763186ffb2341b92e2d8b2547/src/nimblepkg/options.nim#L620

Adding "g" should suffice.


> -g, --global - Creates an old style link file in the special links directory. It is read by Nim to be able to use global develop mode packages. Nimble uses it as a global develop file if a local one does not exist.