added a crude stringinclude feature

Hello,
I added a functions to vibe/templ/diet.d to inject a compile-time generated diet-template into an exsiting template
the code should be fairly obivous. 
Could you review it ?
In return I'll add alot of documentation to vibe.d :D
please discuss at
http://forum.rejectedsoftware.com/groups/rejectedsoftware.vibed/thread/10332/