The goal of this is to be able to adapt the timezone form
in template without hacking the HTML already rendered.
* there are two arrays available:
* `continents` which contains only a list of available continents
* `cities` which contains a list of available timezone cities, associated with their continent
Note: there are two distinct array because RainTPL doesn't support nested loop very well.
All plugins can optionally add an init function named `pluginname_init()` which is called when the plugin is loaded.
This function is aware of the config, and can return initialization errors, which are displayed in the header template.
Note that the previous error system hack no longer work.
