[nem-en] Macros and syntax extensions.

[vc]

> I think the best would be the improved $-string, which could take
> existing StringBuilder instead of creating new one. Some smart syntax
> should be invented for this.

We considered it, but reject this decision. 

1. It's decision not allow override templates.
2. We have not found good syntax.
3. Templates should be recursive and refer one to another (they should have
name and should be callable).

We will be use $-string like syntax, but StringTemplate > $-string and can't
be completely emulated by $-string.

Vlad