Reduce the size of the published package

[futagoza]

This PR simply adds a files field to the package.json so that the size of the published package is reduced by only including the files required (the bin and the dist folders).

Before: https://packagephobia.now.sh/result?p=showdown (All files from this repository included)

After:

$ npm publish --dry
npm notice
npm notice package: showdown@2.0.0-alpha1
npm notice === Tarball Contents ===
npm notice 1.6kB   package.json
npm notice 37.4kB  CHANGELOG.md
npm notice 1.1kB   LICENSE
npm notice 18.8kB  README.md
npm notice 47B     bin/showdown.js
npm notice 173.5kB dist/showdown.js
npm notice 489.0kB dist/showdown.js.map
npm notice 81.1kB  dist/showdown.min.js
npm notice 91.9kB  dist/showdown.min.js.map
npm notice === Tarball Details ===
npm notice name:          showdown
npm notice version:       2.0.0-alpha1
npm notice package size:  183.2 kB
npm notice unpacked size: 894.4 kB
npm notice shasum:        7fa92ad99d8c6962a6eeabe68700a99c454204e4
npm notice integrity:     sha512-pgMk7pliEMzaR[...]Q9wgfrcCdnijg==
npm notice total files:   9
npm notice
+ showdown@2.0.0-alpha1

[tivie]

Thanks