feature request: don't fail on null input or a mode where you just send a comment, no snippet.

[ira-bv]

I'd like to use the same tool to just post with --comment but no file or stdin. right now I just echo | slackcat and it uploads a two link empty snippet. it's good enough not not as aesthetic. if I pipe null slackcat < /dev/null it returns an error. but sometimes a command has no output, and it's not an error. I just want to get the comment without a snippet in such a case, like from a cron job with no output.