This commit adds a new configuration value for the composing section of
settings. Quoting the documentation:
wrap_header_preamble: Option<(String, String)>
optional
Wrap header preample when editing a draft in an editor. This allows you
to write non-plain text email without the preamble creating syntax
errors. They are stripped when you return from the editor. The values
should be a two element array of strings, a prefix and suffix. This can
be useful when for example you're writing Markdown; you can set the
value to ["<!--",\ "-->"] which wraps the headers in an HTML comment.
Manos Pitsidianakis