Add basic arithmetic functions to templating funcmap

Why

I link to grafana dashboards from my prometheus alerts, using some alert labels as grafana variables in the URL to narrow down the dashboard queries. I would like to link to a specific range (eg: 30 mins before/after) around StartsAt and/or EndsAt instead of having to adjust the timeline after opening the link, but this requires basic arithmetic functions (ie: add/sub). In addition, grafana URL params expect unix timestamps in milliseconds, making StartsAt and EndsAt currently unusable, forcing me to return to the alert and manually translate the times from it to grafana via a date picker.

Proposal

Add the following self-explanatory arithmetic functions to DefaultFuncs in https://github.com/prometheus/alertmanager/blob/master/template/template.go

1. add
2. sub
3. div
4. mul

From a user perspective, I don't want to worry about types here, so these functions will require type assertions to differentiate between floats and ints (and maybe strconv.Atoi if string, but I'm not sold on the usefulness of that). Still, this should be relatively simple to implement.

Add the following two functions to allow the use of the above arithmetic functions to manipulate the timestamps in StartsAt and EndsAt:

1. toUnix
2. fromUnix

This is similar to #603 but that request has other concerns about manipulating/accessing current dates, sorting lists etc, so I thought it worth separating the two.