#2184 initial implementation of enhanced plugin pattern

__tests__/src/extend/withPlugins.test.js

__tests__/src/lib/MiradorViewer.test.js

__tests__/src/lib/miradorViewer.test.js

src/extend/index.js

src/extend/pluginStore.js

src/extend/withPlugins.js

src/index.js

src/init.js

src/lib/MiradorViewer.js

src/lib/miradorViewer.js

src/state/createStore.js