Support Gmail's "forward" action (for forwarding emails)

[slix]

Fixes #56

Example:

$ gmailctl diff

--- Current
+++ TO BE APPLIED
@@ -1,14 +1,21 @@
 * Criteria:
     from: rapidrewards@luv.southwest.com
-    subject: "rapid rewards report"
+    subject: "Rapid Rewards Report"
   Actions:
     archive
     forward to: redacted@AwardWallet.com

 * Criteria:
     from: mymileageplus@news.united.com
-    subject: "your monthly statement"
+    subject: "MileagePlus Statement"
   Actions:
     archive
     forward to: redacted@AwardWallet.com

+* Criteria:
+    from: mymileageplus@news.united.com
+    subject: "Your monthly statement"
+  Actions:
+    archive
+    forward to: redacted@AwardWallet.com
+

Do you want to apply them? [y/N]: y
Applying the changes...
$ 

One of my in-Gmail filters was matched exactly, so it doesn't show in the diff.

Gmail now shows 4 filters that have a forward:

Matches: from:(rapidrewards@luv.southwest.com) subject:("Rapid Rewards Report")
Do this: Skip Inbox, Forward to redacted@AwardWallet.com

Matches: from:(mymileageplus@news.united.com) subject:("MileagePlus Statement")
Do this: Skip Inbox, Forward to redacted@AwardWallet.com

Matches: from:(mymileageplus@news.united.com) subject:("Your monthly statement")
Do this: Skip Inbox, Forward to redacted@AwardWallet.com

Matches: from:(DeltaAirLines@e.delta.com) subject:("SkyMiles STATEMENT")
Do this: Skip Inbox, Forward to redacted@AwardWallet.com

[slix]

I forgot to add forward to the README docs.

[mbrt]

Thanks for doing this!

On a first glance everything looks good. A few things to ask:

• Could you please open the PR against the branch cfg_alpha3 instead of master? I'm going to merge another config change in there, so I can do a single config migration, and then merge everything in master and bump the version.
• Please take a look at the failing integration tests. I know they are a bit fragile but they should be fixed before merging.
• Please add at least one config with forwarding rules in the integration tests as well.

[mbrt]

I have included the changes you made to the config file in master, but not your actual implementation. When you are ready you just need to resolve the conflicts and add 1-2 tests and this is good to go (to master directly). No config file change is necessary!

[mbrt]

I have rebased these changes and fixed the tests as well in #75.
Thanks @slix for taking it up to this point!