Support page data declared as named exports

[NMinhNguyen]

I am not entirely sure if you'd like to accept this PR, given the new GraphQL data layer, but I find it a little more pleasant to write export const data = { foo: 'bar' } instead of exports.data = { foo: 'bar' }, but that may just be me 😅

By the way, whilst playing around with this part of the code, I discovered that any assignment to a data MemberExpression gets picked up by load-frontmatter, even things like

const obj = {}
obj.data = {
  seriously: 'yep'
}

Saying that, I thought fixing that might be slightly out of scope for this PR..

[KyleAMathews]

Oh awesome! Yeah this totally should be supported. People naturally expect both forms of exports to be supported and there's no reason to break that expectation.

Thanks for the great PR with tests and all!

Yeah... that is an unfortunate bug. Feel free to fix it if you have a bit more time to contribute.

Also this stuff is still super applicable to 1.0 w/ GraphQL because you'll want to be able to create index pages pulling data from a mixture of JS and say Markdown pages and you'll still want to be able to export static data from your JS pages which could be picked up for the GraphQL query. At some point me or someone else will port all this Babel data extraction code over to 1.0.

[KyleAMathews]

This is out in 0.12.46!