Add greater flexibility in timestamps

[tommyo]

This adds a few features, but is 100% backwards compatible:

1. options can be set on the database rather than individual collections, and can be overridden by collection.

   const db = await RxDB.create({
   name: 'examples',
   options: { timestamps: true }
   });
   const users = await db.collection({
   name: 'items',
   ...
   options: { timestamps: false }
   });

2. options at both levels can accept field name overrides:

   const db = await RxDB.create({
   name: 'examples',
   options: { 
   timestamps: {
     createdAt: 'created',
     updatedAt: 'modified'
   }
   }
   });

Unit tests included.

[coveralls]

Pull Request Test Coverage Report for Build 33

• 18 of 18 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 96.277%

---

Totals
Change from base Build 27:	-0.3%
Covered Lines:	365
Relevant Lines:	371

---

💛 - Coveralls

[rafamel]

Looks good, any chance you can add it to the README so it's documented before merging?

[rafamel]

I've merged for now as I'm making some substantial changes to the project's setup. I've had this module stagnant for a long while and I'm cleaning it up a little. As of now, the tests are failing for replication after the changes introduced in this PR for named custom fields. I would recommend you do a pull from the latest master, npm install, and go from there.

[rafamel]

The issue with replication custom fields are fixed on the latest commit ;)