Channel fixes

Posting to a specific channel does not work unless the data is sent as
a ‘channel’ attribute, not ‘chann’. Also updated the readme to suggest
that channels should be prepended with #.
Add support for attachments

Slack webhooks also support richly formatted messages in the form of
attachments. This allows them to be passed in optionally as the second
parameter.
Make emoji configurable
Merge branch 'emoji'
Merge branch 'attachments'
Merge branch 'channel-fixes'