Re-add Ubuntu support, maintained by the community

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    In #13 you mentioned that you removed Ubuntu support because you'd like to be
    able to do more OS X-specific stuff. However, these many months later
    everything you're doing is pretty cross-platform.
    
    Another reason you mentioned dropping Ubuntu was because nobody at Thoughtbot
    uses Ubuntu. I'm not sure if this has changed, but what _has_ changed is how
    much the open source community loves and uses Laptop for their own computers.
    
    Adding Ubuntu support back in, even with a clarification that it is community
    maintained and not officially supported or used by Thoughtbot, would be a
    wonderful thing. As an "olive branch" of peace between the two Unix operating
    systems, I've forked the repository, added an Ubuntu script (and an easy `zsh`
    installer), and updated the REAME. Would you please consider pulling these
    changes into your repo?
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