Feature Request: Mattermost Notifications

I would love to see support for Mattermost notifications added to Travis CI.

My understanding is that doing this should be pretty easy because Slack notifications are already supported by Travis and Mattermost supports Slack’s webhook format. I believe the issue can be fixed by simply allowing users to change the Slack notification URL as described at https://github.com/travis-ci/travis-ci/issues/6726.

Doing this would allow other Slack compatible services to be used as well as described in the GitHub issue.