Koa 2 support

[jordaaash]

I updated everything to use Koa 2's (ctx, next) => middleware signature.

1. Replaced all generators with async/await
2. Changed this refs to context and fixed args accordingly
3. Replaced var with let/const as needed
4. Removed extends.js (use native Object.assign)
5. Removed co-mocha (mocha has support for async/await)
6. Updated all dependencies
7. Updated all tests (changes above, and popsicle API changed)
8. Updated TravisCI Node versions
9. Added a yarn.lock file
10. Added engines to package.json (async/await requires >= 7.6.0)
11. Updated to version 2.0.0 (I can remove this and rebase before any merge)

[coveralls]

Coverage decreased (-2.5%) to 87.5% when pulling 61597fc on jordansexton:koa-2 into b39f76f on blakeembrey:master.

[blakeembrey]

@jordansexton I don't really use Koa anymore in my own projects. Did you want to take over this module at all? I'm happy to give you access on GitHub and NPM 😄

[jordaaash]

Sure, thanks! We use this in production, so I'm happy to help maintain as long at least as that's the case.

[jordaaash]

What are you working with now, out of curiosity?

[blakeembrey]

Most people want Express.js used in the project, but in personal projects I've been using https://github.com/serviejs (inspired partially by Koa) because it works seamlessly on any environment (I'm focusing on AWS lambda) and for front-end/back-end (client request libraries can use exactly the same interface - it's partially inspired by my work with Popsicle and will eventually make it's way back there too).

I've added you on GitHub and NPM now, it's all yours 😄

Edit: Let me know if there's any issues though.

[blakeembrey]

On a side note also, just be aware that path-to-regexp 2.0 currently has no support for the "asterisk" pattern ('/*'). It's easily worked around and shouldn't be used too often (you would usually want .use instead), but just wanted to make sure you saw that change.

[jordaaash]

Thank you! I'll check over this PR again before publishing. I didn't know that about the lack of wildcard support.

[jordaaash]

And servie looks cool. I haven't seen this yet, I'll check it out.

koa-router has some longstanding issues that haven't been fixed in their koa 2/node 7 branch with path prefixes, layering mounted routers, etc.

My best options looked to be to continue to use kroute, or try one of the trie-based routers, but the ones with koa integration don't seem well-maintained, so I decided to update things here for now.

[blakeembrey]

@jordansexton Feel free to merge and release whenever you want to. I can also transfer the github repo to you entirely, if you'd prefer 😄

[jordaaash]

Thanks for the ping! It's not a problem that it's here, I've just been busy with work. I hope to be able to have time to address this soon. I just didn't want to merge and release if an issue arose that I wouldn't be able to troubleshoot quickly!