New: rule prefer-output-null

[aladdin-add]

fixes #19

[not-an-aardvark]

Oops, I think I might have miscommunicated what the rule should do in #19.

I think it's fine for test cases to not have an output property at all. For example, some rules aren't fixable, and it would be annoying to specify output: null for every test case.

However, the rule should warn when a test case does have an output property specified, and the output property is the same as the code property.

// valid
{
  code: 'foo',
  errors: 1
}

// valid
{
  code: 'foo',
  output: 'bar',
  errors: 1
}

// invalid
{
  code: 'foo',
  output: 'foo', // <-- same as `code`
  errors: 1
}

[aladdin-add]

Hmm... seem like I missed the tests' meaning. don't worry, easy to modify:(

[not-an-aardvark]

Thanks! I have a few nitpicks, but feel free to merge when they are fixed.

[not-an-aardvark]

This example should be updated to have output: 'foo'.

[not-an-aardvark]

The behavior of the rule would be clearer if a sentence was added:

The rule reports an error if it encounters a test case where the output is the same as the code.

[not-an-aardvark]

Nitpick: I think it would be better to change the first word to "Use" rather than "Prefer".