Allow `slack.Send()` to append `MsgOptionBlocks` and not only `MsgOptionText`

[vardan]

Currently slack.Send() is using slack.MsgOptionText to append a body to message. According to Slack community Block Kit is a preferable way to build Slack messages, but having BlockMessage appended in this way (as MsgOptionText) results in Slack not interpreting content of the message correctly and just printing out JSON as-is. Could Send() method be extended to make use of BlockMessage and append it as MsgOptionBlocks? Or, perhaps, you can point me in the right direction if this is not feasable