use methods like += on @ttributes

It'd be lovely to do this:

```
:jabs
  :ready
    $ .concurrency
      &.up
        :click
          $ #concurrency
            @value += 1
            // Or even
            @value++
```

Instead of this:

```
:jabs
  :ready
    $ .concurrency
      &.up
        :click
          $ #concurrency
            @value = @value + 1
```
