Possible to concatenate strings?

I was hoping that this would automagically work?

```elixir
Abacus.eval("concat(a,b)", %{"a" => "hello ", "b" => "world"})
error: undefined variable "var0"

Abacus.eval("a + \" \" + b", %{"a" => "hello", "b" => "world"})
{:error, %ArithmeticError{message: "bad argument in arithmetic expression"}}
```

I don't see any string functions in the source code.
