[tz] Extended data format

[random832 at fastmail.us]

On Thu, Sep 5, 2013, at 13:12, Ian Abbott wrote:
> Include kalimdor
> 
> to include the non-extended zones before it defines the extended zones. 
>   Then zic could be run on either kalimdor or kalimdor-extended, but not 
> both, since the latter includes the former automatically.

What about a require/import statement, which parses the given filename
but does not generate output?

This would also allow us to centralize Rules that are used in more than
one file.