Add support for custom email headers via JSON input (#264)

* Adding custom header with JSON support

* Adding headers in test workflow

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Co-authored-by: W506810_wexinc <spiros.kalogeropoulos@wexinc.com>

README.md

@@ -63,6 +63,8 @@ Some features:
     attachments: attachments.zip,git.diff,./dist/static/*.js
     # Optional priority: 'high', 'normal' (default) or 'low'
     priority: low
+    # Optional custom headers:
+    headers: '{"X-Priority": "3 (Normal)", "X-My-Header": "MyValue"}'
     # Optional nodemailerlog: true/false
     nodemailerlog: false
     # Optional nodemailerdebug: true/false if true lognodem will also be set true