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 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.
Because we may want to scroll up/down with `C-v`/`M-v` or `<next>`/`<prior>` to see more candidates. 100 is a bit small and I just wanted to have a bigger number. To make sure I don't have to worry again, I set it to 500. But this is fine by me if you remove this.