Support for Email highlighting

[RedlineTriad]

Other text editors like neovim support text highlighting for headers, links, and signatures in email files. Full syntax here: https://github.com/neovim/neovim/blob/1186f7dd96b054d6a653685089fc845a8f5d2f27/runtime/syntax/mail.vim

This is very nice when using it for writing emails in a TUI mail client like neomutt.