Option to default to sending plain/html messages

[throwaway-lmao]

Currently, all messages use markdown by default; I can specify something like /plain or /html to change that behavior, but I have to do this for every message manually. I'd like an option to change the default from markdown to plain, or to html.

Additionally, adding /markdown would be useful as a reverse of the current behavior, to explicitly enable markdown for the one message, when it'd otherwise be interpreted as plaintext.