Make compatible with non-oh-my-zsh frameworks

* `${ZSH_CUSTOM}` is only set in [oh-my-zsh](https://github.com/robbyrussell/oh-my-zsh). Updated plugin file to detect where it is installed instead, so it now works with [antigen](https://github.com/zsh-users/antigen) and [zgen](https://github.com/tarjoilija/zgen).
* Add instructions for using the plugin with [antigen](https://github.com/zsh-users/antigen) and [zgen](https://github.com/tarjoilija/zgen).
* Minor reformatting and edit of `README.md` since I was adding sections already.