Allow to run multiple operations (for query batching)

Executing multiple queries in a single query is not part of the spec yet, but [it has been requested](https://github.com/graphql/graphql-spec/issues/375), and several servers already implement it (such as [Apollo](https://www.apollographql.com/blog/batching-client-graphql-queries-a685f5bcd41b/)).

Then, GraphiQL should allow to execute multiple operations against the server. However, when there are multiple operations in the document, the Run button only allows to select either one, but not many of them, or all of them.

Please notice in this screenshot, the 2 operations are expected to be executed together, but that's not possible:

## Proposal

Add a configuration option `"runMultipleQueries"` which, when `true`, if there is more than 1 operation in the document, also adds entry `"Execute all"` in the Run button dropdown: