Use Yarn by default and add support for array values in JS frontmatter

Our 40th patch release on the 0.12 minor series! Woot!

Two nice feature additions from two new Gatsby contributors.

• @donysukardi added support for using Yarn over NPM when creating a new Gatsby site using gatsby new #782
• @Tom-Bonnike extended our JavaScript frontmatter code to support array values. #784 So now the parsing code can extract values in this form:

exports.data = {
  titles: ['My title', 'My other title'],
}