Support $ignore_time in JS library

[ttrefren]

It is not possible to send an engage update that does not modify the $last_seen property of a People record. This is probably a rare use case, but it is annoying not to have this flexibility.

See http spec: https://mixpanel.com/help/reference/http#people-analytics-updates

[tdumitrescu]

Could implement this the same way as mixpanel-node for consistency: https://github.com/mixpanel/mixpanel-node/blob/40d33e1a7e84c674a2165fd97cad0e75a2f4917d/lib/mixpanel-node.js#L367-L370