Update instruction for powerline custom theme

I would like to include `powerline` custom theme inside `unite.vim`. However, it is mixed up with the default json configurations. I do not know how to separate the unite configurations from the other. So I made a repo for the powerline themes I made.

https://github.com/zhaocai/linepower.vim

Do `vimfiler` and `vimshell` have statusline functions like `unite`?