package.json: specify the files to include in the package

[XhmikosR]

Before:

npm notice === Tarball Details ===
npm notice name:          madge
npm notice version:       7.0.0
npm notice filename:      madge-7.0.0.tgz
npm notice package size:  34.3 kB
npm notice unpacked size: 104.0 kB
npm notice total files:   137

After:

npm notice === Tarball Details ===
npm notice name:          madge
npm notice version:       7.0.0
npm notice filename:      madge-7.0.0.tgz
npm notice package size:  13.6 kB
npm notice unpacked size: 44.2 kB
npm notice total files:   10