Use whitelist for published files

[jeddy3]

Closes #28

The dry run looks good.

jeddy3@UNKNOWN postcss-css-in-js % npm publish --dry-run
npm notice 
npm notice 📦  @stylelint/postcss-css-in-js@0.36.4
npm notice === Tarball Contents === 
npm notice 1.1kB  LICENSE                  
npm notice 389B   .prettierrc.js           
npm notice 278B   camel-case.js            
npm notice 11.0kB extract.js               
npm notice 170B   get-template.js          
npm notice 144B   index.js                 
npm notice 416B   literal.js               
npm notice 309B   object-parse.js          
npm notice 7.5kB  object-parser.js         
npm notice 3.3kB  object-stringifier.js    
npm notice 205B   object-stringify.js      
npm notice 172B   object-syntax.js         
npm notice 439B   object.js                
npm notice 421B   template-parse.js        
npm notice 5.0kB  template-parser-helper.js
npm notice 510B   template-parser.js       
npm notice 442B   template-safe-parse.js   
npm notice 623B   template-safe-parser.js  
npm notice 547B   template-stringifier.js  
npm notice 213B   template-stringify.js    
npm notice 1.3kB  template-tokenize.js     
npm notice 297B   un-camel-case.js         
npm notice 2.3kB  package.json             
npm notice 362B   CHANGELOG.md             
npm notice 2.9kB  README.md                
npm notice === Tarball Details === 
npm notice name:          @stylelint/postcss-css-in-js            
npm notice version:       0.36.4                                  
npm notice package size:  12.7 kB                                 
npm notice unpacked size: 40.3 kB                                 
npm notice shasum:        7111c38c2d8bab0790957c6e28729083705167ac
npm notice integrity:     sha512-X/BPDdpfAZeml[...]LnEwyRajjMXbg==
npm notice total files:   25                                      
npm notice 
+ @stylelint/postcss-css-in-js@0.36.4

npm notice 389B .prettierrc.js will disappear once we merge, release and consume stylelint/prettier-config#1