Set patterns for page files to declare metadata

[KyleAMathews]

Markdown has standardized on frontmatter. Don't want to force frontmatter on other file types e.g. JSX.

My proposal for the post-build is that we create a special metadata gathering webpack entry file that pulls in all pages and grabs their metadata. Each loader would need to return a metadata key.

[KyleAMathews]

With React files, using convention currently of looking for a static function named metadata that returns an object. Seems to work well. Thoughts?

[fson]

I needed something like this for HTML files. As an easy to implement solution I've added frontmatter for HTML files: #116

[KyleAMathews]

To make this actionable/finishable – let's let say React components can set a static function called gatsbyData (so doesn't clash with any thing else) which returns a plain JS object and this and HTML/md/yaml/json/etc. loaders should all put their data on page.data.

I'll leave this issue open until this is documented and tested.

[KyleAMathews]

Released in 0.12.23 https://github.com/gatsbyjs/gatsby/releases/tag/v0.12.23