Add automated previews export by SketchTool

[krystof-k]

See SketchTool.

• Export whole Sketch file into one multi-page PDF (is it possible?) as [Sketch filename]/artboards.pdf
• Export all artboards, prefixed to maintain order, as [Sketch filename]/artboards/[page name]/[filename].[ext] in PNG and PNG@2x
• Export all slices, prefixed to maintain order, as [Sketch filename]/slices/[slice name].[ext] in PDF, PNG and PNG@2x