graphql-client: Caching

Problem/Motivation

Support a cache option similar to json-api-client.

Proposed resolution

* Accept 'cache' option
** What should we use as the cache key? A hash of the query? If this is the default, it might be more important that we allow users to override the cache key → .

Remaining tasks

* Implement changes
* Test coverage.
* Documentation updates.

API changes

graphql-client class accepts 'cache' option.