Add option for always show the submode even if noshowmode

Hi, I use powerline so showmode for me is pretty useless but I would like to see if and in what submode I'm in. With this little PR is now possible. The default behavior remain the original one. I wrote just one line in the docs since I'm not use to write vim help file, so let me know if I should add something and primarily if something is wrong in general.