Support for `counter` value with custom type

For example:
```go

type loglvl int

type LoggerPlugin struct {
Config struct {
Level loglvl `name:"log" type:"counter" short:"v" help:"Log level. Higher = more verbose." default:"0"`
} `embed:""`
}
```

Results in:
```panic: reflect.Set: value of type int is not assignable to type cmds.loglvl```

Removing the `counter` type, or setting the type back to `int` works fine.

Is there any way we could add support for counters with custom types as shown in the above example?