imageoptim: Only link ImageOptim.app

Before:

    $ brew cask install imageoptim
    ==> Downloading http://imageoptim.com/ImageOptim.tbz2
    ######################################################################## 100.0%
    ==> Success! imageoptim installed to /opt/homebrew-cask/Caskroom/imageoptim/1.4.0
    ==> Linking ImageOptim.app to /Users/Mathias/Applications/ImageOptim.app
    ==> Linking finish_installation.app to /Users/Mathias/Applications/finish_installation.app

After:

    $ brew cask install imageoptim
    ==> Downloading http://imageoptim.com/ImageOptim.tbz2
    ######################################################################## 100.0%
    ==> Success! imageoptim installed to /opt/homebrew-cask/Caskroom/imageoptim/1.4.0
    ==> Linking ImageOptim.app to /Users/Mathias/Applications/ImageOptim.app
Add Kaleidoscope cask
Add AppDelete cask
Add Pupil Cask
 For more information about the program go to http://pupil.io/
Merge pull request #261 from NanoXD/Kaleidoscope

Add Kaleidoscope cask
Merge pull request #262 from NanoXD/AppDelete

Add AppDelete cask
Merge pull request #263 from NanoXD/Pupil

Add Pupil Cask
Add OmniFocus Cask
Merge pull request #264 from NanoXD/Omni

Add OmniFocus Cask
Add Cornerstone cask.
Add Sidekick cask for managing laptop settings based on locale
Add Slicy cask
Add Things2 cask
Merge pull request #265 from NanoXD/Cornerstone

Add Cornerstone cask.
Merge pull request #268 from NanoXD/Things

Add Things2 cask
Merge pull request #266 from NanoXD/Sidekick

Add Sidekick cask
Merge pull request #267 from NanoXD/Slicy

Add Slicy cask
Add iCHM cask
Merge pull request #269 from NanoXD/iCHM

Add iCHM cask
Merge pull request #259 from mathiasbynens/imageoptim

imageoptim: Only link ImageOptim.app