Remove unnecessary files and folders in published package

[kevinpollet]

Hi,

First, thanks for your good work!

Currently, the published package on npm contains a lot of unnecessary files and folders. To reduce the package size, I think only index.js, bin and lib folders must be included. This PR adds the files property to the package.json to specify the folders and files included in the final package (the file in the main field is always included).

Here is the output of the npm pack --dry-run command:

npm notice === Tarball Contents ===
npm notice 1.4kB package.json
npm notice 5.8kB index.js
npm notice 1.1kB LICENSE
npm notice 7.6kB README.md
npm notice 1.0kB bin/cli.js
npm notice 1.2kB lib/Config.js
npm notice === Tarball Details ===

Here is the output of the npm pack dependency-tree --dry-run command, showing the files included in the published package:

npm notice === Tarball Contents ===
npm notice 1.4kB  package.json
npm notice 70B    .babelrc
npm notice 18B    .deprc
npm notice 28B    .jscsrc
npm notice 89B    .travis.yml
npm notice 5.8kB  index.js
npm notice 1.1kB  LICENSE
npm notice 7.6kB  README.md
npm notice 137B   webpack.config.js
npm notice 1.0kB  bin/cli.js
npm notice 1.2kB  lib/Config.js
npm notice 84B    test/example/amd/a.js
npm notice 67B    test/example/amd/b.js
npm notice 12B    test/example/amd/c.js
npm notice 48B    test/example/commonjs/a.js
npm notice 50B    test/example/commonjs/b.js
npm notice 46B    test/example/commonjs/c.js
npm notice 48B    test/example/extended/a.js
npm notice 48B    test/example/extended/b.js
npm notice 48B    test/example/extended/c.js
npm notice 0      test/example/extended/d.js
npm notice 0      test/example/extended/e.js
npm notice 0      test/example/extended/f.js
npm notice 0      test/example/extended/g.js
npm notice 92B    test/example/onlyRealDeps/a.js
npm notice 78B    test/example/ts/.tsconfig
npm notice 42B    test/example/ts/a.ts
npm notice 30B    test/example/ts/b.ts
npm notice 30B    test/example/ts/c.ts
npm notice 52B    test/example/ts/d.tsx
npm notice 23B    test/example/webpack/aliased.js
npm notice 40B    test/example/webpack/unaliased.js
npm notice 25.0kB test/test.js
npm notice === Tarball Details ===

[kevinpollet]

The tests are failing but I don't think it's related to my changes 🤔

 53 passing (856ms)
  1 failing

  1) dependencyTree
       module formats
         es6
           when given an es6 file using dynamic imports
             includes the dynamic import:

      AssertionError: false == true
      + expected - actual
      -false
      +true

      at Context.ok (test/test.js:643:18)

[mrjoelkemp]

Thanks for this! I fixed that broken test (took a bit of work at several layers). I also went with an npmignore instead of whitelisting certain folders. Appreciate the contribution!