Manage extension configuration

Modules might need to have custom options. This creates the file on `mods-available` and symlinks them to `conf.d`, both at `/etc/php5`.

## To dos

- [ ] Support 5.3.
- [ ] Use variables for configuration paths.
- [ ] Support `extension_name` configuration.
- [ ] Remove extensions we don't manage from `hwr_path_php_mods_enabled`.