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
Merge pull request #259 from mathiasbynens/imageoptim

imageoptim: Only link ImageOptim.app
CONTRIBUTING: Update “adding a Cask” instructions

Ref. #259.
Add explicit link in preparation of it begin required
Merge pull request #272 from vincecima/specify-apps-to-link

Add explicit link in preparation of it begin required
Merge pull request #271 from mathiasbynens/patch-1

CONTRIBUTING: Update “adding a Cask” instructions
Add Carbon Copy Cloner cask
Add Airfoil cask
Add Instacast beta cask
Add SuperDuper! cask
Merge pull request #273 from molawson/instacast

Add Instacast beta cask
Merge pull request #274 from molawson/airfoil

Add Airfoil cask
Merge pull request #275 from molawson/carbon-copy-cloner

Add Carbon Copy Cloner cask
Merge pull request #276 from mfilej/super-duper

Add SuperDuper! cask
Add Synergy cask
Merge pull request #270 from martinp/synergy

Add Synergy cask