Automate releases to git, GitHub, npm

[humphd]

Fixes #404 by modernizing our release process to use release-it. With the changes in this PR, one of @modeswitch or me (or whoever we give rights to), can do:

$ npm run release

This will do a bunch of things automatically:

1. run the tests
2. build dist/
3. bump the version in package.json
4. tag the commit
5. commit everything
6. push the commit and tag
7. create a GitHub Release, with changelog
8. push a new package to npm

See this post for a more verbose description. The tool is interactive by default, and will look something like this:

All default choices are defined in .release-it.json below.

I've included docs in CONTRIBUTING.md, but to summarize what is important:

1. each dev doing releases will need to copy env.sample to .env and include a GITHUB_TOKEN they generate for themselves.
2. npm login so they can publish to npm

I haven't run this fully yet, so I may need to fix a few corner cases. We can do a few runs to get it right, then start using this.

[humphd]

You can see an example of what the GitHub Releases end up looking like here https://github.com/webpro/release-it/releases.

@modeswitch my thinking is to land this, and try doing a release, tweak things, repeat until I get it right. If you disagree, speak up!

My personal view is that I can probably just do minor updates, since I've been able to not break 0.44 versions. However, tons has changed, so I could also do a major number update too. Let me know your thoughts.

[codecov-io]

Codecov Report

Merging #640 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #640   +/-   ##
=======================================
  Coverage   86.62%   86.62%           
=======================================
  Files          16       16           
  Lines        1742     1742           
=======================================
  Hits         1509     1509           
  Misses        233      233

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[humphd]

Merged.