Adds an Npmignore

[orta]

Was making myself a build for the puppeteer v2, and spotted the build was pretty big

Before:

npm notice name:          @orta/backstopjs                        
npm notice version:       4.4.2                                   
npm notice package size:  22.9 MB                                 
npm notice unpacked size: 28.8 MB                                 

After

npm notice name:          @orta/backstopjs                        
npm notice version:       4.4.2                                   
npm notice package size:  660.8 kB                                
npm notice unpacked size: 1.3 MB                                  

This does make some assumptions, so double check the folders which are excluded, but it feels right to me