Add Extensions Feature

[mrshll1001]

This PR responds to the latest SC approval that an extensions array is added to the 360Giving Package Schema (where metadata is):

Property	Title	Description	Type
extensions	Extensions	An array of strings which refer to Extensions in use by this 360Giving data file. Please see the Guidance for details.	Array of Strings

Each item in the array must be a string. I have also used uniqueItems: true to enforce that a publisher cannot accidentally input the same extension twice, this is to support tooling resolving things. If this is out of scope from the SC mandate, I am happy to remove this before merging.

I have confirmed:

• The changes are valid JSON
• The changes are valid JSON Schema (draft 04)
• The JSON schema behaves as expected
  • extensions is not required
  • any non-string element of the array is invalid
  • duplicate elements are invalid e.g. ["dei", "dei"] will throw an error

[mrshll1001]

I have changed the base branch to be 1.4-staging which represents a staging area. #363 (removing unused weight properties from the schema) would represent a PATCH level fix, which would be superseded by this MINOR level change adding new features.

To avoid having to make two separate releases (ie 1.3.1 and then 1.4, or 1.4 and then 1.4.1); if we merge both of these into the staging branch we can merge 1.4-staging into master and then make a single release for 1.4.