exposing pdf asset path

I needed to be able to reference the path of an asset in the view:
```
style: "background-image: url('#{wicked_pdf_asset_path('pdf-line-bg.png')}')"
```
I didn't find a good way to do that in the docs. I found the relevant place where image_tags are generate and found exactly what I needed. I extracted it into a method and exposed it...