Helm cleanup

The code of prelude-helm module, with ugly formatted code and many typo and grammar mistakes, is really a mess. I know helm is a gigantic package, but it does not require that much code to configure. Many options are not necessary and most of the keys are subjected to personal taste. I think most of the code in the helm module is too personal to make it a part of prelude, we should leave most of the customisable options to the users, and have a bare minimal working configuration like this one:

https://gist.github.com/toctan/ac523ef379f14f5486e2 

In this pull request, I removed all the options which are already the default and thus are not necessary and fixed some typos.
I don't think many people use `picture-mode` or `artist-mode`.