Snippet Organisation

## Snippet Organisation

Allow sub-directories within snippets with arbitrary depth.

For example:

```
## My header

Markdown copy..

{{snippet: my_sub_directory/my_snippet}}

More copy...
```
Where `snippet_name` is in a `snippets/` directory in the flow root with a `.txt` extension, eg `my-flow-name/snippets/my_sub_directory/my_snippet.txt`.

**Considerations**
- File extension must be whitelisted within InputFile
- DirectoryInput  Doesn't allow directory traversal
- Easily extensible to any future DirectoryInput type like outcomes
I'm not sure file extension makes much difference to Smartdown, it's only going to try parse it as Smartdown. As the behaviour is to just ignore the file, that's potentially confusing for an author. 

I think we might be better off without the extension check. Keep it simple?