- ensure that width/height of canvas are trimmed down after drawing a
  purple rectangle in the center

- add test script that uses jest
  - (ci): change CI to run test and test:pub
- configure jest and babel-jest
  - add coverage/ directory to gitignore
  - add babel-jest@23 for Babel 6 support
    - and configure it for .js files due to a jest bug
  - add .babelrc to configure babel-jest
    - can't use .babelrc.js as babel-jest@23 doesn't support it
      - jestjs/jest#5324
    - can't use .babelrc for babel-loader (have to duplicate config)
      because babel-loader@6 has some bugs with it and babel-loader@7
      doesn't support webpack@1
      - babel/babel-loader#552

(deps): add jest, babel-jest, and canvas-prebuilt to devDeps
- add canvas-prebuilt@1 to support jest's jsdom v11
  - canvas is used by jsdom for, well, canvas interactions

- so it's not just handling one single square in the middle, make the
  shape more complex to ensure algorithm works properly

- currently returns a 1x1 canvas... should probably be 0x0, but that
  requires some refactoring and may be a breaking change if anyone
  is relying on that behavior
  - reliance on that behavior may certainly be possible given how long
    this library has gone without a release, though no one's reported
    that as a bug either
  - it's a pretty big edge case, so I'm not sure that it matters much
    either way tbh

- also, this gets us to 100% code coverage!!

- change CI to output and upload coverage reports

- (docs): add dat slick coverage: 100% badge to README

- standardize w/ the rest of my libraries and better parity between
  test/ and src/

- none of these variables are ever re-assigned
  - and new standard linter versions error on that
  - I wrote this a while ago when ES6 was new and just used `let` for
    everything instead of `var` at the time. Now ofc I use `const`
    everywhere and `let` when needed
    - also replace `var` in for loops with block-scoped `let`

- change docs to use const as well

- basically the same one I use in all my other libraries
  - I believe it's an old version of Node output from gitignore.io iirc
- it has comments unlike the old one, so that's much nicer for those
  who aren't totally sure what each entry is for