Flask - add support for custom execution_context_class argument

[alvinchow86]

I am using Graphene with the Flask graphql integration. Graphql core supports an argument in its execute() function called execution_context_class, among other things as well. I would like to pass a custom ExecutionContext class but it doesn't look like there is a way to do this here in the Flask server integration (and probably the other ones as well).

My use case is that I'd like to use the new UnforgivingExecutionContext added recently in graphene (see graphql-python/graphene#1255), so that I can have better exception handling and Sentry integration in my application.

It seems like this should be an easy fix, I may look into submitting a PR shortly.