Adds support for own toolbar and uploader templates

Hi @ejholmes
I came across this great gem. But for my client its very important to customize the UI. So I just added the possibility to configure the templates "toolbar" and "uploader". I also added a description.
What do you thing about this?

## Custom template paths
The toolbar and the uploader are client side templates (ejs)
To customize these you can set custom paths your very own templates.
__Its important that the template has a different name then (toolbar or uploader)!__

```ruby
ActiveAdmin::Editor.configure do |config|
  config.template_paths = {
      toolbar: 'path to my custom toolbar template',
      uploader: 'path to my custom uploader template'
  }
end
```
__Please check the original templates for an example__
app/assets/javascripts/active_admin/editor/templates

### Make your templates available
Unfortunately sproket can not find templates from your
asset folder when requireing them in a gem. So you have to
make your templates manually avaliable.

```javascript
//= require_tree ./path_to_your_templates
```