Generalise/Document adding icons for custom tasks

I have kick-started things here with a very simple way of allowing other modules to define icons for routes for which the module doesn't provide any icons. A simple hook to alter the route data and instructions to add CSS.

Thanks Gaurav, could you please also update the README to include this in within your MR?

The Readme is included in MR. I tested the MR and it works like expected.

Thank you for your work!