Require target.from as fallback, add replyTo support with sensible defaults

[DrSleez]

• Added support for replyTo on incoming requests.
• Made target.from required and used it as a fallback when no from is provided in the request.
• If replyTo is not provided, it defaults to the effective from (field from if present, otherwise target.from).
• Request field from continues to override target.from.
• Updated README to document required/optional fields and new defaults.

Why

• We needed replyTo and noticed there was an open issue requesting the same feature.
• Making target.from required ensures a reliable fallback and avoids sending emails without a valid sender.

Behavior

• from: request field overrides; otherwise fallback to target.from (required).
• replyTo: optional; defaults to the effective from when not supplied.

Docs

• README updated: targets and request fields grouped into Required vs Optional; replyTo behavior documented.

Migration

• Existing targets must now include from (required). Add a default like no-reply@example.com if missing.

Closes

Closes #9

[Feuerhamster]

Thank you, but your PR messed up some spacings. Can you fix it? I will merge it then.

[DrSleez]

Hey, sure could you tell me which spacings you mean? It was probably my prettier addon which messed something up.