add gatsby-starter-barebones and gatsby-starter-barebones-preact (#819)
add gatsby-starter-barebones and gatsby-starter-barebones-preact (#819)

* add gatsby-starter-barebones and gatsby-starter-barebones-preact

* add demo links for gatsby-starter-barebones and gatsby-starter-barebones-preact

Khaledgarbaya/gatsby,gatsbyjs/gatsby,fk/gatsby,0x80/gatsby,mingaldrichgan/gatsby,Khaledgarbaya/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,mickeyreiss/gatsby,chiedo/gatsby,ChristopherBiscardi/gatsby,chiedo/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,0x80/gatsby,mingaldrichgan/gatsby,chiedo/gatsby,0x80/gatsby,mickeyreiss/gatsby,fabrictech/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,fk/gatsby,danielfarrell/gatsby,danielfarrell/gatsby,okcoker/gatsby,okcoker/gatsby,fabrictech/gatsby,mickeyreiss/gatsby,gatsbyjs/gatsby,fabrictech/gatsby,fk/gatsby,danielfarrell/gatsby,okcoker/gatsby,ChristopherBiscardi/gatsby,Khaledgarbaya/gatsby