Add CTFE execute function

[Chang Long]

CTFE execute will be very useful on web develop, for example It 
is very hard to create a CTFE version template engine with rich 
feature. But we can use execute call to transe template file to d 
code string, then mixed it to application.

The vibed project is very cool and I want to add my Jade template 
to this, but I can't find other way to do it.

Maybe we also need a compiler option to enable this CTFE io 
operate because security.