Composer working dir

Hi!

In our deployment setup we need to change the _working_dir_ in which composer is running on. Normally this is done by the `--working-dir` argument but I think it is better to change that within the `within()` function of the :run  task.
So I made a separate configuration-value to change the working dir. Default is set to the current `release_path` as before

Maybe this should be public available for all :)
Can you use `fetch` here?

```ruby
set :composer_working_dir, -> { fetch(:release_path) }
```