Fix warning message of s:config() function is found in plugconf files

[tyru]

Got

[WARN] parse warnings in /home/tyru/volt/plugconf/github.com/leafgarland/typescript-vim.vim:
* s:config() is deprecated. please use s:on_load_pre() instead

It is painful to rewrite s:config() to s:on_load_pre() by hand. #214

Expected

[WARN] parse warnings in /home/tyru/volt/plugconf/github.com/leafgarland/typescript-vim.vim:
* s:config() is deprecated. please use s:on_load_pre() instead, or run "volt migrate plugconf/config-func" to rewrite existing plugconf files