version 0.3 breaks image captions

[aarongervais]

Hi,

After upgrading from 0.2 to 0.3, the showCaptions option of gatsby-remark-images stops working. e.g.:

{
  resolve: `gatsby-remark-images`,
  options: {
    showCaptions: ['title'],
  },
}

The version of gatsby-remark-images doesn't seem to make a difference, neither does changing its configuration options. Downgrading back to gatsby-remark-relative-images 0.2 fixes the issue.