Do not run the add color command if the color scheme is already installed

I was annoyed with the notice always showing up every time I run boxen, so I added a check to not run the command if the color scheme is already present.

I'm not completely comfortable with the hardcoded plist file, but Plistbuddy only accepts a plist path and I did not find a command to output the plist path from `defaults -app iTerm` or other command.

Hopefully this looks good, otherwise please suggest any improvements!