Allow `this` modification in `no-mutation`

As proposed by @scottnonnenberg in https://github.com/jhusain/eslint-plugin-immutable/pull/15, the no-mutation rule could accept mutations to this, as they may make sense in some applications.

To do: Accept mutation of the this object, only if a corresponding flag is set.

[jfmengels]

Hmm, not sure it makes sense. If you only allow this to be mutated in the constructor, then you don’t really need it, and you can do it with closures.

function Person(age, firstName, lastName) {
  const isOver18 = age > 18;
  return {
    age: age,
    fullName: function() {
      return firstName + ' ' + lastName;
    }
  };
}

If you have an other use-case for this, I’m happy to hear it 😃

[deepu105]

@jfmengels I was about to open a new issue then I came across this thread, I found this plugin useful and I guess being able to configure mutation of this only inside constructor is somthing thats useful in react apps especially, as you woul often assign this.state = .. in constructor but you wouldnt want to do that outside the constructor as state shouldnt be mutated. so somethink like below might be bit more flexible

"fp/no-mutation": ["error", {
  "commonjs": true,
  "allowThis": true,
  "allowThisInConstructor": true,
  "exceptions": [
    {"object": "foo", "property": "bar"}
  ]
}]

or even better would be if we can just specify method in the exceptions

"fp/no-mutation": ["error", {
  "commonjs": true,
  "allowThis": true,
  "exceptions": [
    {"object": "this", "property": "state", "method": "constructor"}
  ]
}]