feat($parse): Custom private field tagging

[lrlopez]

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

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

Some examples:

myModule.config(function($parseProvider) {
  // disable private fields restriction
  $parseProvider.restrictFieldsMatching(null);
});

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.

Closes #4509

[lrlopez]

By the way, I've already signed the CLA (is Mary Poppins on holidays?)...

I've tried two different approaches to this PR. The first one stored the match pattern into the $parseProvider.options object, but it didn't pass some non-related tests. After some digging, I found when you instantiate a parser without using $parseProvider the pattern option won't be defined. This forced me to check inside ensureSafeMemberName() if the pattern is defined so I'd use the default one otherwise. Since the function call execution time is critical, I don't think that is a good approach.

The actual PR uses the second approach: the pattern is declared on a local variable which defaults to /^_|_$/ and can be changed using $parseProvider. This works on every case. Also, code changes are way simpler and the matching test is fast.

[laurelnaiad]

For some reason I can't see any commits in the PR... am I underprivileged or is there a missing commit?

[lrlopez]

Uh! Something strange happened... it was there yesterday! I've just forced the push again, let's see if it's solved now

[laurelnaiad]

Cool. Hopefully this can go into a quick 1.2.1.