Improve violation reporting location in no-missing-placeholders

[bmish]

Fixes #276 (half of it).

Previously, the violation might not even be reported inside the specific context.report() call that it was for.

Now, we report on the data object when it exists.

module.exports = {
  meta: {
    messages: {
      foo: 'hello world', // violation should not be reported here
    }
  },
  create(context) {
    context.report({
      node,
      messageId: 'foo', 
      data: { // violation should be reported here
        bar: 'abc' 
      }
    });
  },
};