Add sniff codes for array assignment shortcut

[sirbrillig]

This replaces the UndefinedVariable sniff code with UndefinedArrayVariable if the variable is an "array push shortcut" (I don't know of a better name for it): $x[] = 5;

Also, when marking such a variable, this sniff considers that variable defined at that point going forward. (This will prevent future uses of the same variable from being reported as undefined.)

Fixes #98