Added colorization to Scrolls logs

This solves the problem with hard to read logs.
It solves it by colorizing the output of the logs.

New feature relies on the gem Rainbow.

To toggle colorization provide the following to Scrolls

  Scrolls.enable_colors = true
  Scrolls.colors = {:custom_key => :green, :custom_value => :blue}

Rainbow: https://github.com/sickill/rainbow