Change 'attach_keyword' option from string to regex

Change 'attach_keyword' option from string to regex

A regular expression is much more useful than a single word to check for
forgotten attachments, especially when writing in multiple languages.

This is inspired by the muttng patchset abort_noattach [1], which I use
for years, but newly implemented to make minimal changes to the neomutt
source code.

The keyword is no longer mentioned in the error message as regular
expressions tend to be longer.

The option 'attach_keyword' could be renamed 'attach_remind_regex' as in
the muttng patch.

[1] https://github.com/codito/muttng-patchset/blob/master/patches/muttng.abort_noattach.diff

Closes: #138