Improved `rev` handling for PutAttachment()

It is now possible to pass a rev argument to the PutAttachment() method by way of options, instead of the named rev argument, as well. I really really think this is the last one of these 😖

Improved `rev` handling for DeleteAttachment() method

It is now possible to pass a rev argument to the DeleteAttachment() method by way of options, instead of the named rev argument, as well.

Improved `rev` handling for Delete() method

It is now possible to pass a rev argument to the Delete() method by way of options, instead of the named rev argument.

Adds the Client.Membership() method

Client.Membership() can be used to query the /_membership endpoint on clusters. See https://docs.couchdb.org/en/latest/api/server/common.html#get--_membership

Documentation updates

This moves most of the examples and documentation from the wiki into the godoc.

No meaningful changes here, I'm only publishing the release so that pkg.go.dev will see the updates.

Fix bad merge

The merge from master to v3 branch resulted in an accidental v4 import path. This corrects that.

Partitioned queries, and multi-query resultsets

This release adds two major features:

1. Support for partitioned queries, which is now provided by CouchDB 3.x
2. Support for multiple queries to a view. This support has been around for ages (partial support all the way back to 1.x), but was never supported by Kivik. Now it is!

Breaking changes:

• The Find, Explain, CreateIndex, GetIndexes, and DeleteIndex methods now take optional (variadic) kivik.Options arguments. This should not break normal usage, but may pose a problem if you depended on the exact function definition (such as in an interface definition) in any code.

Fix changes panic

This release fixes a panic when attempting to access LastSeq, Pending or ETag on a changes feed before the first iteration.

More consistent error formatting

Previously, formatting a kivik-generated error with %v produced a different output than calling the Error() method. This version standardizes output for both methods.

Test suite updates

No functional changes. This just updates the test suite to run against CouchDB 3.0.0, and Go 1.14.