Solves issue #29 and improves PR #18

[BePo65]

This PR updates PIG to enable customization of ProgressiveImage and thereby extend the html created within each <figure> tag.

I did not merge all my commits to make it easier to review the changes.

Here is what I did:

Refactoring

• dropped support for pre IE8 css styles (market share is so small)
• fixed base address of images in html pages in /test directory
• changed source code to use arrow functions instead of anonymous functions (current technology)
• changed source code to use es modules (current technology, supported by all browsers an node LTS)
• migrated code to es6 (using classes) to enable customization of ProgressiveImage

Features

• made ProgressiveImage customizable (customization is done by creating a derived class)
• added demos with basic and with customized ProgressiveImage in directory /test using
  source file
  umd version
  packed esm version
• updated README.md to include new API elements

Tooling

• switched from gulp to esbuild to create umd version and packed es module from source
• switched from gulp to eslint (with airbnb styles)
• did not migrate gulp watch function, but could easily be added

BREAKING CHANGE

I moved code used for production systems to /dist (umd) or /dist/esm (esm) directory, to make it easier to publish the packages to npm.
But besides the new location of the code, using the basic (uncustomized) version did not change.