Add an email helper

[jeanconn]

We haven't had much in the way of a general utility package for a while, so brainstorming ideas, it would probably be useful to toss something like

https://github.com/sot/acdc/blob/067ec76ed344d67a80ee739ea1e0ce204c9cbaa2/acdc/common.py#L29

in here. I used it again in aca_hi_bgd and it seems pretty universal.

[jeanconn]

Granted, then you start to wonder if aca_helpers is going to be imported by everything, should it use local imports or whatever so we don't get import scope creep.