snippets: changed selection behaviour of for cycle

first selection - all `array` variables, second selection all `i` variables
(previous behavior: first selection - first `array` var, second selection - second `array` var).

I have ran into some errors when i was tabbing from 1st selection to 2nd selection but it was already reported issue (https://github.com/emmetio/emmet-atom/issues/171) related to 3rd party plugin (Emmet). Works fine with Emmet turned off.