Customization tweaks

This goes hand in hand with the recent addition of the [Customization wiki](https://github.com/LFDM/oh-my-zsh/wiki/Customization)

* Updates the README with a link to the wiki
* Refactors the customization examples in the `custom` folder

TODO:
@ncanceill, there's also a change included that goes a bit against your PR #2645

I removed ZSH_CUSTOM from `oh-my-zsh.sh` and set it directly in the `zshrc.template`, including a comment that refers to the wiki.
We need to decide which approach we like better and add another commit to this PR before it can be merged. 

Closes #2295