Add strictness level for plusplus

[ttsirkia]

It would be nice to have an option for plusplus which allows use cases such as

for (var i = 0; i < myArray.length; i++)

or

myVar++;

But it should complain about this:

myVar = myOtherVar + mySecondVar++;

So I suggest that there are three modes: false, strict/true or simple that allows usage when not in complex expressions.

Especially that looping case is so common that there should be option to allow it although it would be elsewhere forbidden.

[jugglinmike]

I think there's a case to be made for the safety of increment/decrement operators when used as statements. @rwaldron What do you think?

As for the actual implementation, given that plusplus: true enables the restriction, I think the most logical configuration value (that also maintains backwards compatibility) would be plusplus: expr.

[ttsirkia]

I'm fine with any option names. My original suggestion was that true means the original and the most restricted mode and the new option (such as expr or simple) allows usage in certain places.