[Fix #456] Allow end user to customize Prezto and ZSH

Introduce .zprezto.before and .zprezto.after conventions
  * If .zprezto.before exists, its .zsh files are sourced before Prezto is initialized
  * If .zprezto.after exists, its .zsh files are sourced after Prezto is initialized
  * .zprezto.before files are for customizing Prezto such as calling zstyle
  * .zprezto.after files are for general ZSH customization outside the scope of Prezto