Configurable follow-on check intervals

Adds support for letting checks provide an optional check interval to
use once a check has failed. i.e. if the usual check has an interval of
300, check every 30 seconds once its failed. When the check clears
return to checking every 300 seconds.

Closes #18
Merge pull request #21 from pandemicsyn/fast-follow-on

Configurable follow-on check intervals