feat($parse): Custom private field tagging

Commit 3d6a89e introduced the concept of private field that
can not be evaluated on an Angular expression.

This PR allows changing the pattern that tags a field as
private. It also allows disabling the private field access
restriction.

Some examples:
```js
myModule.config(function($parseProvider) {
  // disable private fields
  $parseProvider.restrictFieldsMatching(null);
});
```

```js
myModule.config(function($parseProvider) {
  // Now private fields will be those beginning with 'priv'
  $parseProvider.restrictFieldsMatching(/^priv/);
});
```

Custom private fields allow backwards compatibility with
AngularJS <1.2 projects that use underscores in scope fields
(i.e. data retrieved from a MongoDB instance) while still
enjoying the new private field security policy.