Add multiple mattermost channel support

[chengweiv5]

This patch based on below PR

https://github.com/NotSqrt/mattermost-integration-gitlab/pull/3

which add multiple mattermost channel support.

To add a webhook to a project, add url like below

http://this-project-server:port/new_event?mattermost_webhook_url=

Signed-off-by: Chengwei Yang [email protected]

[NotSqrt]

That would be really nice if you could update the tests and the docs to reflect this new feature !