Port this package to Flask

[hxuanhung]

Hi, I've ported this package successfully to Flask and I want to create a PR for it.

Particularly, I make two changes:

• from graphene_django.views import GraphQLView to from flask_graphql import GraphQLView
• and request_type = request.META.get("CONTENT_TYPE") to request_type = request.mimetype

It's not clear for me to make this works for both Flask and Django. Do you have any idea?

[lmcgartland]

I think the best thing might be to have to modules exported from this package, a view for Django and one for Flask. They can share the underlying logic. File a pull request just so I can take a look and we can work together on getting it merged.