Support for omni graffle

[cforce]

Please add support for graffle diagrams

image::../graphics-originals/diagram1.graffle["Hierarchy of building blocks"]
asciidoctor: WARNING: could not embed image: diagram1.graffle; image file is an unrecognized format

image::../graphics-originals/diagram1.graffle.png["Hierarchy of building blocks"]
asciidoctor: WARNING: image to embed not found or not readable: :../graphics-originals/diagram1.graffle.png

[pepijnve]

Interesting idea, but not trivial to do. We would have to automate omnigraffle's export functionality using AppleScript or something like that.