Added backend option to coordinator.

Now it would be possible to change the backend during depends on task:

```ruby
on roles(:app), backend: ::SSHKit::Backend::CustomBackend do
  puts self.class # => SSHKit::Backend::CustomBackend
end
```

Also thinking to add support get backend by symbol, if you'd like:

```ruby
on roles(:app), backend: :custom_backend do
  puts self.class # => SSHKit::Backend::CustomBackend
end
```