[add] short_path formatter in section_c

[update] test

autoload/airline/formatter/short_path.vim

@@ -0,0 +1,8 @@
+scriptencoding utf-8
+
+function! airline#formatter#short_path#format(val) abort
+  if get(g:, 'airline_stl_path_style', 'default') ==# 'short'
+    return '%{pathshorten(expand("'.a:val.'"))}%'
+  endif
+  return a:val
+endfunction