changelog bot

I was thinking it might be useful to create a GH changelog bot. Whenever a GH release is pushed (e.g. `npm tag`) it'd look at all the commits that were part of it, and create a changelog as the body of the github release. This would make it easier to look at what changed between versions, credit contributors, and improve the overall quality of the project.

It'd be super neat if someone would be willing to take this on! Any ideas on how to further improve this / approach this would be very welcome!

## Stuff that'd probably be good to list in a changelog
- links to all commits
- link to corresponding PR
- link to corresponding commit
- link to who made a commit
- comment on all PRs that were merged into the release, linking to the release
- link to contributors that participated in making the patch possible (comments, merges, reviews, etc.)
- link to first time contributors that participated in making the patch possible
- time since last release

## Stuff that probably shouldn't be part of these automated changelogs
- any human text 🤖 – long form patch notes, and introductions to features are probably best left to dedicated blog posts, introductions, or docs updates. Leaving the option out all together is probably

## Stuff that'd be neat if we could pull off
- go back in time, and craft notes for _all_ releases on the `choojs` org (automated, of course)

## Final thoughts

Before closing this out, I thought it might be fun to create an example changelog of what I was thinking, for the latest Choo release. (Sorry in advance for the mentions, @silawrenc :P)

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# [2017-09-09, Version 6.4.1](https://github.com/choojs/choo/releases/tag/v6.4.1)
This release contains 1 commit, was released by @yoshuawuyts, and comes 3 days after the last release. Many people came together to create Choo v6.4.1. We couldn’t have done it without all of you. Thanks!

### Commits
- [[`8595f1f`](https://github.com/choojs/choo/commit/8595f1f765cf1e76d5106d4a5eb3a740477aab49)] – Populate state.query before calling router (Simon Lawrence) [#575](https://github.com/choojs/choo/pull/575)

### First Time Contributors
- [Simon Lawrence (silawrenc)](https://github.com/silawrenc) :tada:

### Contributors
- [Yoshua Wuyts (yoshuawuyts)](https://github.com/yoshuawuyts)