* Refactor tests to use stubs for nvim-web-devicons
* Simplify getting icon
`get_icon` does not need the 2nd parameter (`ext`) as it will derive it
from the given `name` (1st argument) itself.
* fix component(filetype) proper icon based on filetype
With https://github.com/kyazdani42/nvim-web-devicons/pull/125 a filetype
based lookup was implemented upstream which can be used to get an icon
when a file doesn't have an extension.
fixes#578