allow arbitrary data to be stored alongside the rest of the document

[missinglink]

This PR addresses the issue mentioned in pelias/pelias#747, also known as 'custom data'.

This change would allow the operators of a Pelias instance to store arbitrary data alongside any document.

The addendum entries are 'namespaced' and encoded as strings before being saved to elasticsearch.

paired with:

• https://github.com/pelias/schema/pull/342
• https://github.com/pelias/api/pull/1255