Support enums

I'm seeing `Error::UnsupportedFeature("enums")` when attempting to deserialize a simple enum:

```rust
#[derive(Deserialize)]
enum InstallType {
    BasicMsi,
    InstallScript,
    BasicMsiWithInstallScript,
    Unknown,
    InstallScriptUnicode,
}
```

With this ini:
```ini
[Startup]
ScriptDriven=1
```