tmi.js: Incorrect number of months on subs and resubs

Actual behaviour: “sub for 9 months” shows 0 for the number of months in the resub and sub events

Expected behaviour: It should show 9 months

A fix would be to change the property from msg-param-months to msg-param-cumulative-months, but I am not sure if this would work for all cases as I have yet to run into each case where months is provided.