Cleanup the plugin

[cyrilwanner]

The plugin grew quickly which is very nice but it should now be cleaned up/refactored a bit so it still is easy to understand, to maintain and to add more functionality.

• Split the index.js file into multiple smaller files, maybe one per loader and their options
• Use babel to use modern js features while still supporting node 6 and to make the code in svg-runtime-generator.js a bit less verbose (use jsx and don't write React.createElement code)
• Add tests for the newly added functionalities (the new config options, svg sprites and the runtime generator)
• Add contributing guidelines, issue & pr templates
• Add code linting

Some feature improvements:

• if outputPath is specified and relative, make sure it uses the same path on the server & client
• svg sprites contain all sprites of all pages in the memory of the server, we should only include the sprites on the current page
• provide a component for the webp fallback