chore(changelog): create and add script

[levithomason]

Resolves #18.

The changelog and changelog commit in this PR were generated using the script. See commit:

docs(changelog): updated by levithomason [ci skip]

Generate / Update

› npm run changelog 

...generating changelog (be patient)
...committing updated changelog
Done.  You can now 'git push' the updated changelog.

No Changes

When the the changelog is already up to date and the script is run:

› npm run changelog

...generating changelog (be patient)
CHANGELOG.md already up-to-date.

Future PRs

Later, this script can be integrated into the CI process. The repo/branch/user can be inferred from CI env vars at that time as well. This just gets it started.

[nfischer]

Later, this script can be integrated into the CI process.

SGTM. I like the idea of doing it manually for the time being. When the manual way gets to be too much of a pain, we can automate it.

Just tried this out, and it seems pretty good. The only thing I don't like is that it tries to automatically push the commit, which I wouldn't expect. I'm fine with auto-generating the commit though. If @ariporad disagrees, I'm fine with going with majority opinion on this though.

Also, I think we should try to keep PR titles to the commit format @ariporad suggested, since that's what the final commit message gets based off of after the squash-merge (correct me if I'm mistaken). That's only if we're going to continue with that format for commit messages.

[levithomason]

I'm down with manual process, automating only if necessary. Also good with no automatic push, I will remove it.

The squash message is editable at merge time. Clicking the merge button switches it to an "edit message" input.

[levithomason]

Done. Also, just realized the PR titles will become part of the changelog as well! Updating...

[nfischer]

^ good to know. I feel like github's changing everyday, it's sometimes hard to keep up with it all

[levithomason]

EDIT removed, ignore the rambling...

[nfischer]

LGTM

[levithomason]

Codecov is gagging on this PR for some reason, can't rerun. Going to merge since it touches no functional code.