Accurately represent Matrix messages containing newlines in Slack

Newlines don't seem to be replicated correctly in slack when sent from matrix as text. Set the `PreserveWhitespace` field in the format `Context` to preserve newlines as expected. Without this field, all `"\n"`s are stripped from the text during parsing, resulting in some odd-looking messages in slack. Before: (matrix)  (slack)  After: (matrix)  (slack)