Can't Specify Markdown Boolean in PostMessageAsync()

[Luke-Robinett]

When using the PostMessageAsync method and including some markdown in the text argument, the markdown is not parsed, resulting in the markdown itself being present in the Slack message. In reviewing the API docs, the mrkdwn parameter should be true by default so I'm not sure why this isn't working, but it would be nice to have access to the mrkdwn boolean in this signature so I can make sure it's set to true. Also, I'm only using the markdown features that are supported by Slack per the docs. Let me know if there's some other way of going about this. Thanks.