Add API to get icon from the value of 'filetype'

[Bakudankun]

I want to get icons from the value of &filetype of buffers, e.g. cpp, help, qf, dosbatch etc. to indicate what filetype is set to the buffer. I'd be glad if this plugin have such API.

[lambdalisue]

It's not difficult but quite tough thus PR is welcome 👍

Maybe it's a good idea to starts from https://github.com/lambdalisue/nerdfont.vim/blob/master/autoload/nerdfont/path/extension.vim