Feature Request: Add "Configuration" property to NuGetPackSettings to specify build configuration

Feature Request: Add `Configuration` property to `NuGetPackSettings`which would be passed to `nuget.exe` as `-Prop Configuration={NuGetPackSettings.Configuration}`

When building a NuGet package for a project NuGet will use the default configuration from the project (e.g. `SomeProject.csproj` defaults to it's `Debug` configuration–when running `nuget.exe` will expect to find the build output in `bin\Debug\`).

Currently the workaround I'm using is to specify:

```
NuGetPack(project, new NuGetPackSettings {
...
ArgumentCustomization = args => args.Append("-Prop Configuration=" + configuration)
});
```

This works fine, but having `Configuration` as a setting property would be convenient and cleaner:

```
NuGetPack(project, new NuGetPackSettings {
...
Configuration = configuration
});
```

Locate NuGetPackSettings and the NuGetPack entry point, then trace how settings become nuget.exe arguments. Verify that a Configuration value is accepted and passed as -Prop Configuration, while preserving the existing ArgumentCustomization workaround behavior.

Written by the indexing model from the issue text.