Feature: Enable customization of content tag with html options

Currently it's not possible to specify custom HTML attributes for field's the content tag. For example:

```erb
<%= editable(@article, :title, id: "my-custom-id") %>
```

will output:

```html
<span class="editable" data-id="my-custom-id" data-model="article" data-name="title" data-placeholder="Title" data-type="text" data-url="/articles/1" data-value="My first article" title="Title">My first article</span>
```

This PR adds an `:html` option to the `editable` helper that can customize the content tag's HTML attributes. The same view code up there will output:

```html
<span class="editable" id="my-custom-id" data-model="article" data-name="title" data-placeholder="Title" data-type="text" data-url="/articles/1" data-value="My first article" title="Title">My first article</span>
```