Tweaks and modifications

A summary of the changes:

- Added global option `:as_user` for all installers (`Sprinkle::Installers::Installer`) to run commands as specific user (`su -c '....' -s /bin/sh #{@options[:as_user]}`)
- Added `:sudo` as global option for all `Sprinkle::Installers::Installer` classes
- Updated readability of the `push_text` and `replace_text` installers