Add gen_tags.vim plugin support

Display a "Gen. gen_tags" text in section x when gen_tags
is generating tags, similar to gutentags support.

autoload/airline/extensions/gen_tags.vim

@@ -0,0 +1,19 @@
+" MIT License. Copyright (c) 2014-2020 Mathias Andersson et al.
+" Written by Kamil Cukrowski 2020
+" Plugin: https://github.com/jsfaint/gen_tags.vim
+" vim: et ts=2 sts=2 sw=2
+
+scriptencoding utf-8
+
+if !(get(g:, 'loaded_gentags#gtags', 0) || !get(g:, 'loaded_gentags#ctags', 0))
+  finish
+endif
+
+function! airline#extensions#gen_tags#status(...) abort
+  return gen_tags#job#is_running() != 0 ? 'Gen. gen_tags' : ''
+endfunction
+
+function! airline#extensions#gen_tags#init(ext) abort
+  call airline#parts#define_function('gen_tags', 'airline#extensions#gen_tags#status')
+endfunction
+