Removed `parse` parameter

[Aliance]

If you want Slack to treat your message as completely unformatted, pass parse=full. This will ignore any markup formatting you added to your message. https://api.slack.com/docs/formatting#parsing_modes

So I just completly removed parse parameter.

To test my changes I've created a messages.txt file:

first plain text message
second message with <http://google.com/|link>

And run cat messages.txt | go run slackcat.go twice (in master and in my branch). This is the results: The first 2 messages - master, the last 2 - with my changes.

cc @paulhammond

Fixes #12

[Aliance]

@paulhammond any response?