RFC: Added do-expression syntax for regex matches.

This trivial patch allows one to write regex matches like

```julia
julia> match(r"(.+)@(.+\.[A-Za-z]{2,4})", "julia@julia.org") do id, dom
         println("id: $id")
         println("domain: $dom")
       end
id: julia
domain: julia.org

julia> match(r"(.+)@(.+\.[A-Za-z]{2,4})", "julia_at_julia.org") do id, dom
         println("id: $id")
         println("domain: $dom")
       end

julia> 
```

Also adds a short description to the manual.

(Inspired by @StefanKarpinski's match macro comment in #1448.)

Comment: I wanted to write `do (id, dom)`, but julia didn't seem to like that.  That would be more readable to me...