Add list of files to be published on npm

[easingthemes]

Expected Behaviour

Only core files are published as npm package
Add list of files in package.json

"files": []

Actual Behaviour

All files are published as npm package.

Unpacked Size
382 kB

This creates false assumption of a big package and probably will stop a lot of people from using it in client side JS.
Core files are minimal, size will drop x 100

[meryllblanchet]

Thanks for raising the issue @easingthemes !
Assigning to @shazron for feedback - your PR is welcome.

[shazron]

PR #13