Is it possible to support `./a`, `./a -f` and `./a -f 3` at the same time? (i.e. optional flags with optional values)

I'm in the process of getting to know this library. It's quite nice so far, thanks for writing it!

***

I tried using `Maybe (Maybe Int)`

and I got:

No instance for (ParseField (Maybe Int))
arising from a use of ‘Options.Generic.$dmparseRecord’
• In the expression: Options.Generic.$dmparseRecord @MainOptions

I also thought to model this via:

```hs
data OptFlagOptValue a
= NoFlag
| Flag
| FlagValue a
```

But not sure how I could translate using the above data type into a clean API that supports the following:

```
$ command
$ command --flag
$ command --flag 123
```
Am I missing something? Or is this not supported (yet) perhaps?