Support arbitrary additional data to be passed to post_message
Support arbitrary additional data to be passed to post_message

The underlying `slacker` Python library supports additional argument types, like attachments={...}.

This PR captures that data from command-line and passes them to slacker.

Ex:
    echo 'oh noes!' | slacker-cli -n failbot -c ops attachments='[{"color":"red", "fallback":"oh noes!"}]'

juanpabloaj/slacker-cli