Improve Exim forward pipe generation

[gbp]

Allow JSON to be more explicit instead of automatically prefixing Exim forward pipes with the run-with-rbenv-path wrapper.

New format would look like:

{
  "email": {
    "email-user": [
      { "script": "path/to/default/script", "suffix": "" },
      { "script": "path/to/suffix/script",  "suffix": "suffix" },
      { "script": "path/to/ruby/script",    "suffix": "ruby", "wrapper": "rbenv" }
    ]
  }
}

This will allow us to set the script to bin/rails actionmailbox:..., using the new "rbenv_exec" wrapper. The old doesn't allow this as it calls source instead of exec.

[gbp]

@sagepe I've pushed a couple of fixups and an extra commit which adds an option to chdir before the wrapper is executed.

Tested on staging and this works with the old style JSON and the new version:

diff --git a/deployments/vhosts/alaveteli/staging.alaveteli.org.json b/deployments/vhosts/alaveteli/staging.alaveteli.org.json
index de7a809d9..af6814900 100644
--- a/deployments/vhosts/alaveteli/staging.alaveteli.org.json
+++ b/deployments/vhosts/alaveteli/staging.alaveteli.org.json
@@ -29,11 +29,11 @@
       "foi-staging-secondary"
    ],
    "email": {
-      "foistaging": {
-         "": "bin/rails.sh action_mailbox:ingress:exim URL=https://staging.alaveteli.org/rails/action_mailbox/relay/inbound_emails INGRESS_PASSWORD=123",
-         "team": "script/handle-mail-replies",
-         "track": "script/handle-mail-replies"
-      }
+      "foistaging": [
+        { "suffix": "", "script": "bin/rails action_mailbox:ingress:exim URL=https://staging.alaveteli.org/rails/action_mailbox/relay/inbound_emails INGRESS_PASSWORD=123", "wrapper": "rbenv_exec" },
+        { "suffix": "team",  "script": "script/handle-mail-replies", "wrapper": "rbenv" },
+        { "suffix": "track", "script": "script/handle-mail-replies", "wrapper": "rbenv" }
+      ]
    },
    "exec_before_down": {
       "user": [

[sagepe]

I think this all looks good @gbp - can you squash the fixups, etc, so this is good to merge and we can get it into a release with another small change I'm working on.