Publish to NPM

[edrpls]

It would be great to distribute the library as CJS and/or ES modules through npm.

Currently, this is blocked by #32, as image preload was originally thought for UMD without considering either CJS or ES modules.

We should publish the transpiled modules instead of the source code as this may require extra configuration for our users depending on the builder/transpiler of their choice (looking at you, Webpack/Babel).

We haven't had the use-case for either module yet, but it would be great to have the option for 2.0.0.

See:

#32, #63 and this method