Potential Alertmanager plugin improvements

[stylianosrigas]

• No proper colouring on alert levels. All alert levels (info, warning, high, critical) are shown as red colour which makes focus quite hard
• Message should be bigger and separate from the other entries in the attachment to make it clear from the first look what the issue is
• Info like Generated by a [Prometheus Alert] is of no real use and makes the alert more noisy

[cpanato]

thanks for the Issue! are you willing to PR that? i will be happy to review