Slack Incoming Webhooks instructions / interface has changed

[liyang]

Now all incoming requests go to hooks.slack.com, and the hook setup instructions doesn't refer to a ‘token’ any more.

I removed the 'channel' and 'username' options because this script cannot mirror to than one Slack channel at a time—nor does the channel / bot username ever change—and in any case can be configured via the defaults on the Slack hook configuration interface when setting one up.

Also, if a mapped nick speaks on IRC, the 'from' variable wasn't being mapped to the corresponding Slack nick.

Also, I improved the nick mapping by only matching on word boundaries.

Also, I took the liberty of adding support for '/me derps.'

Also, I decided to replace the [#irc-channel] tag from the text sent to Slack, since we do not monitor more than one channel per instance. Instead, we tags private messages to the bot as such.

Feel free to take any or all of the above. Or let me know which changes you're not sure about and I'll remove them from this PR.