Added recursive image search to image gallery

[johnsbuck]

Resolves Issue #20 in adding recursive search for image gallery.

To test, you can do the following:

1. git clone https://github.com/johnsbuck/obsidian-image-gallery.git
2. git checkout issue-20
3. npm run build
4. Copy main.js from dev folder to <vault_folder>/.obsidian/plugins/obsidian-image-gallery
5. Test with example query below:

```image-gallery
path: path/to/img/super/folder
type: vertical
recursive: true
```

[kyleyoungblom]

This works great, thanks! Really useful addition.