[enh] Issue 1673 - Add template keyword to `is` expression TypeSpecialization

http://d.puremagic.com/issues/show_bug.cgi?id=1673

This was a feature request by @andralex . Paging @WalterBright too since it's a language change.

It adds the `template` keyword for **TypeSpecialization** in **is** expressions. It will allow these comparisons:

```d
is(Alias == template)
is(Alias ident == template)
```

See also the test-case. The spec will be updated in another pull if/when this pull is merged.