[Feature]: Add time length to down state of monitored host

[aloskam88]

Describe the feature you would like to see

Would be nice to specify "I only want to know if this host is down for more than X amount of time".

Describe how you would like to see this feature implemented

Update existing down state notification to also allow for a time length. Also add option to alert if down more than X amount of times in Y time frame.

[henrygd]

This will be in the next release, see #625