Add ability to set In-Reply-To tag (#81)

* Add ability to set In-Reply-To tag

This is useful to reply to a specific email, e.g. to a patch sent by
email on a mailing list.

Please note that both the In-Reply-To and the References tags are
filled-in. In theory, only the In-Reply-To tag should be enough but they
are both linked to the same idea and in theory most emails readers
should support a Message-ID given in In-Reply-To tag. So just in case,
we fill both to imitate many email clients.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

* Update test.yml

Co-authored-by: Dawid Dziurla <dawidd0811@gmail.com>

README.md

@@ -42,6 +42,8 @@ Some features:
     bcc: r2d2@example.com,hansolo@example.com
     # Optional recipient of the email response:
     reply_to: luke@example.com
+    # Optional Message ID this message is replying to:
+    in_reply_to: <random-luke@example.com>
     # Optional unsigned/invalid certificates allowance:
     ignore_cert: true
     # Optional converting Markdown to HTML (set content_type to text/html too):
@@ -59,7 +61,7 @@ Some features:
 Gmail security settings may cause this Action to fail. This failure may involve a message in the GitHub Actions details about access being denied and an email from Google to the email account being used about a sign-in being blocked and why.
 
 Changes in Gmail settings may be necessary to get this action to work.
-1. Google treats this method of using email as a "Less Secure App". However, "Less Secure Apps" can be enabled in Google profile settings. There doesn't appear to be a static link for this, but if you go to Google profile settings while signed-in and type "less secure apps" into the search bar, the appropriate instructions will come up.
+1. Google treats this method of using email as a "Less Secure App". However, "Less Secure Apps" can be enabled in [Google profile settings](https://myaccount.google.com/lesssecureapps).
 2. IMAP needs to be enabled in Gmail settings as described [here](https://support.google.com/mail/answer/7126229?hl=en).
 3. If the Gmail account you're trying to use in this Action is already 2FA (Two Factor Authentication) enabled, the 2FA password will need to be provided as well, which isn't included in the default template.
 

action.yml

@@ -44,6 +44,9 @@ inputs:
   reply_to:
     description: An email address that will appear on the Reply-To field
     required: false
+  in_reply_to:
+    description: The Message-ID this message is replying to
+    required: false
   ignore_cert:
     description: Allow unsigned/invalid certificates
     required: false

main.js

@@ -51,6 +51,7 @@ async function main() {
         const cc = core.getInput("cc", { required: false })
         const bcc = core.getInput("bcc", { required: false })
         const replyTo = core.getInput("reply_to", { required: false })
+        const inReplyTo = core.getInput("in_reply_to", { required: false })
         const attachments = core.getInput("attachments", { required: false })
         const convertMarkdown = core.getInput("convert_markdown", { required: false })
         const ignoreCert = core.getInput("ignore_cert", { required: false })
@@ -80,6 +81,8 @@ async function main() {
             cc: cc ? cc : undefined,
             bcc: bcc ? bcc : undefined,
             replyTo: replyTo ? replyTo : undefined,
+            inReplyTo: inReplyTo ? inReplyTo : undefined,
+            references: inReplyTo ? inReplyTo : undefined,
             text: body ? getBody(body, false) : undefined,
             html: htmlBody ? getBody(htmlBody, convertMarkdown) : undefined,
             priority: priority ? priority : undefined,