Support channel override

Changes

* Rename `webhook` to `webhook_url`
* Support channel override

## REF
slack

> #### Channel Override
Incoming webhooks have a default channel, but it can be overridden in your JSON payload. A public channel can be specified with "channel": "#other-channel", and a Direct Message with "channel": "@username".

slack-notifier

> Once a notifier has been initialized, you can update the default channel and/or user.
https://github.com/stevenosloan/slack-notifier#setting-defaults