is_list()

[soyapark]

Similar to #75, we need an addtional feature of Contact entity which returns boolean if this message is from a mailing list. (there is some request from our CHI paper survey)

We can figure this out by parsing message data but we can't tell it by solely looking into email address. We can add another attribute to contact or parse one of message sent by this contact and determine if this is list or not on-demand.