ExcelDnaPack: Add support for packing any kind of resource

Relates to #329.

It would be useful if ExcelDnaPack could be adapted to allow any kind of file to be embedded in the `-packed.xll` file.

We could include a new element e.g.`Resource` to the `.dna` schema that allows the user to specify the file path along with some (optional) metadata (e.g. `Type`), combined with an API exposed by `ExcelDna.Integration` that gives the developer access to the resources packed.

e.g.

```xml

```

---

```csharp
public static class ExcelDnaUtil
{
/// Returns the names of all the resources packed in the XLL.
/// An enumerable that contains the names of all the resources.
public static IEnumerable GetPackedResourceNames() { ... }

/// Loads the specified resource from the XLL.
/// The case-sensitive name of the resource being requested.
/// The resource; or if the resource does not exist in the XLL.
public static Stream GetPackedResourceStream(string name) { ... }
}
```