Mail header matching and overriding

[neoice]

I see your TODO lists "Mail header matching and overriding."

I want to +1 that 😃

if I wanted to take a stab at implementing this as a PR, do you have any tips on where I should start, coding standards to follow, etc?

[mhale]

One way to look at it is that the Subject field is in email headers, so header matching is just a generalised form of subject matching.

My plan for implementing matching was to add a MatchHeader function on Filter, with corresponding test coverage. After that, refactoring the subject matching to use the general header matching function would be simple. Storage would require key/value pairs underneath a "Headers" string map on Filter. I was going to leave the Subject field in the UI though, as IMHO it is more user-friendly to have it.

I haven't got a plan for overriding the headers on a Route though.

As far as coding standards goes, I just try to keep functions small, and run go fmt on the files before I commit them.