Multi-element directives should explicitly declare themselves as being multi-element

With regards to https://github.com/angular/angular.js/issues/5370, I think it's a bit crazy to completely disable the use of attributes/directives ending with `-start` or `-end`.

I would propose something like this:

```
angular.module('blah', [])
  .directive('multiElementDir', function() {
    return {
      multiElement: true, // Explicit declaration
      // ...
    };
   });
```

And, instead of this:

```js
              var directiveNName = ngAttrName.replace(/(Start|End)$/, '');
              if (ngAttrName === directiveNName + 'Start') {
                attrStartName = name;
                attrEndName = name.substr(0, name.length - 5) + 'end';
                name = name.substr(0, name.length - 6);
              }
```

we could have something like

```js
              var directiveNName = ngAttrName.replace(/(Start|End)$/, '');
              if (hasDirectives.hasOwnProperty(directiveNName) && hasDirectives[directiveNName].multiElement) {
                // Only worry about start/end names if directive is explicitly multiElement
                if (ngAttrName === directiveNName + 'Start') {
                  attrStartName = name;
                  attrEndName = name.substr(0, name.length - 5) + 'end';
                  name = name.substr(0, name.length - 6);
                }
              }
```

then we could allow both multi-element attributes and attributes ending with `-start` and `-end`, which I think would be nice.