Allow Treat::Core::Installer.install to accept schiphol download options, eg. suppress progress bar for some terminals

I was not able to run the installer on my Chef-based AWS Opsworks -based Rails application deploy, it would fail with 
    tput: unknown terminal "unknown"

Suppressing the progress bar fixed this, so I changed the install method signature to allow passing in options to the downloader.

Sorry this is my first pull request: not sure how to exclude my README.md file changes without rebranching or something.