Transform the value of an argument

Hello,
Is there a way to transform the value of an argument like:

```kotlin
val parser = ArgParser(args)
val file: File by parser.option(ArgType.String, transform = { File(it) })
```
instead of
```kotlin
val parser = ArgParser(args)
val filename: String by parser.option(ArgType.String)
val file: File = File(filename)
```

Start by tracing the ArgParser.option API, including ArgType.String and its existing argument conversion behavior. Done means the shown transform lambda can produce a File value directly from the option and the behavior is covered by tests.

Written by the indexing model from the issue text.