Changes to the way oh-my-zsh sources custom scripts

This is my suggested change to the behaviour of oh-my-zsh main script. It will now first look what scripts have been specified in $ZSH/custom. If they happen to have the same name as scripts in $ZSH/lib, then they are sourced instead (i.e. custom files now override oh-my-zsh settings). I decided to do this so that I can have direct control over what oh-my-zsh does to my shell without modifying your default script files. If this is not a functionality you are looking for, feel free to ignore the pull request.

Secondly, custom plugins now go directly into $ZSH/custom/<myplugin> directory instead of $ZSH/custom/plugins/<myplugin>. The script has no functionality to source anything in any other $ZSH/custom subfolders, so why have the plugins subfolder anyway?