Style the hero portion of Middleman landing page

* Add `Fira` fonts to directory
* Add new color variables for Middleman branding
* Install Bourbon, Neat, and set CSS to very basic
* Edit hero content in copy
* Update hero with relevant blocks of content before CSS style of terminal
* Add browser using code
* Delete old, unused stylesheets from main CSS file
* Uninstall Bourbon and Neat files
* Update project with Bourbon and Neat gems installed through Gemfile