Date augmenting and ranges

[jasonvarga]

When testing out another date range issue, I was using the 3.1 branch, and noticed I had broken date ranges.

This is because I added augmentation to the date fieldtype, but didn't include support for ranges.

This PR adds support for augmenting date ranges, explains the breaking change in the changelog, and adds support for querying date ranges in GraphQL.

range {
  start(format: "Y-m-d")
  end(format: "Y-m-d")
}