Merge branch 'ashmaroli-feature/jekyll-data'

Use jekyll-data to read data files from theme-gem

Use a plugin to read data files from gem

  - jekyll-data v1.x
  - jekyll 3.5 automatically requires a theme's runtime deps
  - jekyll v3.5.0 had bugs. Hence use v3.5.1 and beyond

Add `rake preview` task for testing `/test` during theme development

Remove Rakefile

Add rake task for testing theme during development
- `bundle exec rake preview`