Add support for using stdin in place of the message flag

[atc0005]

This could potentially be applied by first implementing #469 and then checking for - as the filename.

The goal would be to accept piped input and use that instead of using the value passed to the message flag.

Since we'd be making it easier to push bulk content into this app we'd need to consider truncating to a safe input limit (since Teams enforces those). The truncation could be applied automatically or by request. If automatically, then add a flag to disable truncation (letting the submission attempt fail if too large).