New cask: omni-graffle-standard

New cask: omni-graffle-standard
As omni-graffle is the pro version, requiring a different
license, omni-graffle-standard installs the standard
version of the omni-graffle software.
Added extention to omni-graffle-standard link

Added extention to omni-graffle-standard link
Merge pull request #846 from derphilipp/omni-graffle-standard

New cask: omni-graffle-standard