[RFC] Implement Partitions api

We are currently adding partition support to CouchDB https://github.com/apache/couchdb/pull/1605
This document details how I think the api for partitions should work for nano.

Partitions allow a user to store related documents into a `partition` within CouchDB. Using the new partition endpoints this would then allow a user to query only documents in a specific partition. This leads to much faster query time as CouchDB only needs to fetch the documents from a subset of the shards for a db.

To store a document in a partition a user prefixes an id with the partition name e.g `{_id: "partition1:my-doc", "field": "one"}` and `{_id: "partition2:my-doc", "field": "one"}`.

Then using a tradional view to query the document you would use these endpoints:
Map/Reduce:
`/my-db/_partition/partition1/_design/my-view`

And for Mango:
`/my-db/_partition/partition1/_find`

The idea around partitions, which I've hopefully conveyed really quickly above, is that data in a partition is quite separate and when a database is partitioned a user would work with each partition separately. I would like to reflect that kind of thinking in the api. So I propose that we would add a new function called `partition` which accepts a partition name and returns an object for you to query a specific partition. Hopefully the below example explains it.

```js
await nano.db.create('db1', {partition: true});
const db = nano.use('db1');
await db.insert({
views: {
aview: {
map: "function(doc) {\n if (doc.group) {\n emit([doc.some, doc.group], 1);\n }\n}",
reduce: "_count"
}
}
}
}, '_design/example-query');

db.insert({some: "field"}, 'partition1:doc1');
db.insert({some: "field2"}, 'partition1:doc2');
db.
// This goes in partition 2
db.insert({some: "field2"}, 'partition2:doc1');

const partition1 = db.partition('partition1');

// This will only return doc with id `partition1:doc1`
const docs = await partition1.find({
selector: {
some: "field"
}
});

const docs2 = await partition1.view("example-query", "aview", {include_docs: true});

const partition2 = db.partition('partition2');
// A view can be used for each partition
const docsFromPartition2 = await partition1.view("example-query", "aview", {include_docs: true});
```

The new partition object would support all the `.find` and `.view` options to query with and internally would remember the name of the partition to use when querying.

Currently we don't support _all_docs or changes.