Add New Icons Or Custom Icon Support

[munadutta]

I started using this app today and find out most of my accounts doesn't have icon, icons are missing in the app, and there also no option to add custom icons to the accounts, so please add this feature this is very important.

[jp-weber]

2fast uses SVG files for the icons which are obtained from the repository of simple-icons. The easiest way to add missing icons is to submit them to simple-icons: https://github.com/simple-icons/simple-icons