Ignore log patterns

[codekoala]

This builds on the work done in #270. It allows alerts to be ignored if the pod logs contain certain patterns. In my case, I was regularly receiving alerts about pods (for 3rd party applications) that terminated themselves when they lost a leader election. Since this is a normal part of their operation, I don't need to be alerted about this event every time it happens.

Changes proposed in this pull request:

• add a new ignoreLogPatterns config option
• compile those patterns into regular expressions
• check to see if pod logs match any of the specified patterns

[abahmed]

Thanks :-)