introduce option to add favicons for different browsers and apps

[stillbeben]

We may want to use different favicons for different browsers and apps. One way to generate those is the favicion Generator by DeGraeve.

https://github.com/rullmann/pelican-theme-flex/pull/2/commits/7474daf51f110f8e58a4c96258ba5de3297f4590

Maybe you'd like to cherry pick my commit to introduce this feature?

[ashusengar]

If this issue is still open I would like to do it. Admin please assign me this issue.

[alexandrevicenzi]

Go ahead.