Add support for client that uses GET as transport mechanism (#186)

Current implementation always uses POST as the transport mechanism. Adding GET support enables usage of GET queries for caching simply via URL.

Some notes:
- I left the existing API for creating a new client as is, but the implementation could be much cleaner by introducing some sort of configuration struct when creating a new client
- The construction of the query parameters follows the logic from Apollo's client implementation, which can be found here https://github.com/apollographql/apollo-client/blob/8beb4820edc6352996e08f7f73bde3573f1eb666/src/link/http/rewriteURIForGET.ts
- Updated integration tests to use both sets of clients. Updating the tests to use a test suite would be cleaner

docs/CHANGELOG.md

@@ -29,6 +29,7 @@ When releasing a new version:
 
 - genqlient can now run as a portable binary (i.e. without a local checkout of the repository or `go run`).
 - You can now enable `use_extensions` in the configuration file, to receive extensions returned by the GraphQL API server. Generated functions will return extensions as `map[string]interface{}`, if enabled.
+- You can now use `graphql.NewClientUsingGet` to create a client that uses query parameters to pass the query to the GraphQL API server.
 
 ### Bug fixes: