Add formatter for short paths

Add new formatter that shows the file path up to the first parent directory

autoload/airline/extensions/tabline/formatters/short_path.vim

@@ -0,0 +1,19 @@
+" MIT License. Copyright (c) 2013-2019 Bailey Ling et al.
+" vim: et ts=2 sts=2 sw=2
+
+scriptencoding utf-8
+
+let s:fnamecollapse = get(g:, 'airline#extensions#tabline#fnamecollapse', 1)
+
+function! airline#extensions#tabline#formatters#short_path#format(bufnr, buffers)
+  let _ = ''
+
+  let name = bufname(a:bufnr)
+  if empty(name)
+    let _ .= '[No Name]'
+  else
+    let _ .= fnamemodify(name, ':p:h:t') . '/' . fnamemodify(name, ':t')
+  endif
+
+  return airline#extensions#tabline#formatters#default#wrap_name(a:bufnr, _)
+endfunction