Support rejection rules

[eXeC64]

It'd be nice if nanolist could have some custom rules for rejecting emails to lists based on an external script.

For example:

• Reject [PATCH] emails where the patch has been corrupted (broken whitespace)
• Reject emails that top-post
• Require subjects to be formatted a certain way
• Banning the word "generics" from a golang mailing list

nanolist would not implement such logic itself, but would allow messages to be piped into an external command. If the return code is 0, the message is posted to the list. If it is 1, the stdout of the script is sent as a reply to the sender.

[ddevault]

Reject HTML emails