Make module context-aware

[mattolson]

In order to load the module in multiple contexts, such as in a parent thread and a worker thread, we need to be context-aware.

Fixes #10

References

• https://nodejs.org/api/addons.html#addons_context_aware_addons
• worker threads throws exception while loading npm lib nodejs/node#21783 (comment)

[schroffl]

Have you tested it? I don't doubt that you did, but I'd be rather safe than sorry.
If you want to, you can also add a Context-Awareness test using mocha. Otherwise I can take care of it, though I am not sure when. It's not a necessity and I will still publish a fix tomorrow, but future-proofing is always a good idea :)

Thank you very much for all your contributions so far!

[mattolson]

Yes, I tested it locally using the code posted here: #10 (comment)

I will add a test to the PR as well.

[mattolson]

@schroffl I have added a test to demonstrate that the module will load correctly twice from the same process. This test fails with Error: Module did not self-register if run against master.