Add .npmignore file to reduce npm package by 90%

[KaliedaRik]

This repo publishes a package to NPM manually (using npm publish on the local command line). As a result the publish step will generate and push a tarball which only excludes files determined by the .gitignore file, resulting in a package sized 33.0MB (56.7MB unpacked). The package includes all the files in the docs and demo folders:

Developers do not need the documentation and demos when all they want to do is include Scrawl-canvas in their projects. This PR adds an .npmignore file to exclude these folders from the publish step. The resulting package size is 1.8MB (5.6MB unpacked):