TS support for custom icons

[hanselabreu]

I understand that:

• I'm submitting this PR for reference only. It shows an example of what I'd like to see changed but
  I understand that it will not be merged and I will not be listed as a contributor on this project.

Since this library allows custom icons, we should allow accepting string as the prefix and iconName.

[drusellers]

I just ran into this trying to use my custom uploaded icons. Thankfully I could use fak but for the icon name I had to use an existing one so TS would be ok. I would def like to see some love for custom icons in the library.