allow passing parameters to custom panels

currently the panel-functions are 0-arity. To write reusable panels, there should be a way to pass parameters to them. Few suggestions:

## 1) pass the start! options to panels

```clj
(defn permissions-view [{:keys [permissions]}]
...)

(dev-tools/start!
{:state-atom re-frame.db/app-db
:permissions #{:a :b}
:panels-fn {:permissions {:label "Permissions" :fn permissions-view}}})
```

## 2) like 1, but just one key, e.g. :data

```clj
(dev-tools/start!
{:state-atom re-frame.db/app-db
:data {:permissions #{:a :b}}
:panels-fn {:permissions {:label "Permissions" :fn permissions-view}}})
```

## 3) something else

Start at dev-tools/start! and trace how :panels-fn currently invokes 0-arity panel functions. Compare the proposed ways to pass start! options or a data key, then establish which API should be supported. Done means custom panels can receive parameters through a documented, agreed interface.

Written by the indexing model from the issue text.