Redefined blocks now actually work as expected

Fixes #79. I'm not 100% sure that this doesn't break anything but from my tests it seems to be fine.

Basically makes this work as you would expect:

`base.ect`
```html
<< block 'image' : >>
  <img src="11111111111.png" />
<< end >>
<< include 'imageBox' >>
<< block 'image' : >>
  <img src="2222222.png" />
<< end >>
<< include 'imageBox' >>
```
`imageBox.ect`
```html
<div class="imageBox">
  << content 'image' >>
</div>
```

The above example previously would combine both image boxes to the 1111111111.jpg block content, though now it outputs correctly.

I'd imagine something like this could be used to make #50 work as well without too much pain.