Added support for numerical ngTrueValue and ngFalseValue attributes

feat(ng): support for numerical ngTrueValue and ngFalseValue attributes

Checkbox input fields may now be declared with numerical ng-true-value and ng-false-value settings.
The input's corresponding model will be interpreted as a number rather than a string representation.
Regular string configurations which do not parse as numbers will be treated as before.

BREAKING CHANGE: numeric values for ngTrueValue and ngFalseValue attributes are
now interpreted as numbers rather than their string representation. This may affect
cases where a string representation of a number is expected in a checkbox field's
associated model and the identity operator (===) is used for comparison.

To migrate the code, make comparisons with the numerical typed value or use the
equality operator (==) in place of the identity operator to ensure type invariant value
comparison.

Before:

```
    if(scope.value === '1') {
      ...
    }
```
After:
```
    if(scope.value === 1) {
      …
    }
    // or
    if(scope.value == 1) {
      ...
    }
    // or
    if(scope.value == '1') {
      ...
    }
```