Suggestion: Use create-react-context for 16.0+ compatibility

[mAAdhaTTah]

Currently, mdx-tag requires React 16.3+. I believe this is mostly due to the user of the new createContext, a feature landed in React 16.3. For my project, I need to use React 16.2 because Enzyme, the testing library I'm using, doesn't yet support the new features added in 16.3, so I'm using React 16.2 in my project to run my tests.

If mdx swapped out its usage of createContext for the create-react-context library, I'd be able to continue to use 16.2 in my tests.

I'd also be willing to do this work, if this is something you'd accept.

[johno]

It makes sense to support all React v16 since it looks like a pretty trivial change, I'd gladly accept a PR to use create-react-context.

[mAAdhaTTah]

Opened PR #179 for this.

[johno]

Published a prerelease with these changes, 0.13.1-0. Please let me know if you encounter any issues.

[mAAdhaTTah]

No issues, this works, thanks!