Be able to have more contentBlock in template

Hi,

I have the following usecase :
```hbs

{{> Template.contentBlock}}

```

So now I can call the `template` like this:
```hbs
{{#Component}}
Part Head
{{else}}
Part Content
{{/Component}}
```

So it's fine even if it's look "hacky" to use `else` block to provide two `contentBlock`.
In case I built something with more than two I can not use this trick.

I wish to be able to define multiple `contentBlock` in template and to call be able to specific which content for which `contentBlock`.

Example usage :
```hbs
{{#Component}}

Part Head

Part Content

{{/Component}}

and for the definion:
```hbs

{{> Template.contentBlock1}}

```

It should be like extending the `Template.elseBlock` to allow more than one.
The goal is to be able to build more flexible template.

Start by tracing how Template.contentBlock and Template.elseBlock are implemented and how template blocks are parsed and rendered. Compare the current two-block behavior with the requested named contentBlock1 and contentBlock2 usage. Done means templates can define and render more than two distinct content blocks without using else.

Written by the indexing model from the issue text.