Mattermost alerting

[mr-gosh]

As you support Slack - is there an integration of mattermost planed too?
As I don't want to post problems to external websites...

[jamesrwhite]

I wasn't planning to add it myself but the alerting code is very simple if you fancy having a go 👍

[zodman]

mattermost and slack webhook are compatibles!

[jamesrwhite]

@zodman great! I'll leave this open still as I'd still like to add a config option for it to make it clear it is supported but if it can re-use the same code as Slack that's awesome.