Add better error for '=+' typo

[sderickson]

from @alexholman, copied from codecombat/codecombat#3710

Related to my initial issue report: codecombat/codecombat#3709
I found my problem was typing =+ instead of += for an increment, resulting in an infinite loop condition.

This is not a very informative error for this easy typo. The code should catch the use of =+ and warn, much like it catches capitalization issues in object names and missing ':' in loops.

[UltCombo]

I guess most of you already know this, but a problem is that =+ is actually valid syntax. For example, in JavaScript:

var x = '5';
x=+x; // Convert string to number
hero.say(typeof x); // number
hero.say(x); // 5

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But still, maybe Aether can show a custom JSHint warning for this specific construct?