Adding unary operator + to transform string into numbers

When reading the test files, I spot that `parser.spec.js` contained:

```
it('should parse unary - expressions', () => {
  expect(evaluate("-1")).toEqual(-1);
  expect(evaluate("+1")).toEqual(1);
```

So I divided them in two tests: 
1. should parse unary - expressions
2. should parse unary + expressions

I realised that the unary `+` can be used to transform expressions of any type into numbers returning a number or NaN, see [Mozilla Dev Network](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Arithmetic_Operators?redirectlocale=en-US&redirectslug=JavaScript%2FReference%2FOperators%2FArithmetic_Operators#-_.28Unary_Plus.29 "Mozilla Dev Network").

Therefore I implemented `PrefixPlus` in the way `PrefixNot` has been implemented and wrote some test for that:

```
    it('should parse unary + expressions', () => {
      expect(evaluate("+1")).toEqual(1);
      expect(evaluate("+'1'")).toEqual(1);
      expect(evaluate("+'not a number'")).toEqual(NaN);
    });
```

However I am not sure if the angular team wants to add this level of complexity.