Add custom color palettes to iterm plist

[sykesm]

Various custom palettes from github are added to the default plist for
iterm. 'Solarized Dark' remains the default palette but it can be
overriden based on sprout properties to something like 'arthur'.

[hiremaga]

I might concerned about the complexity this adds to the existing recipe. If you'd like a similar change let's chat in person to see if we can find a different approach.