Add -ignore-initial-run flag and update changelog

[nikkictl]

Signed-off-by: Nikki Attea nikki@sensu.io

Closes #5
Closes #6

[echlebek]

If -ignore-initial-run is true, there is no reason to read through the file. The program could just initialize the state file and exit. Can you look into doing that? It would likely speed up the first execution quite dramatically. :)

[nikkictl]

@echlebek I used the presence of the state file, rather than the offset, to determine if the plugin is on it's first run. Let me know if this approach is more in line with your suggestion.

[nikkictl]

I worry it doesn't address the number of matches described in #6 (comment) though. We'd have to read through the log file to determine the initial matches.

[nikkictl]

I've set the state file's offset on the first run with -ignore-initial-run to the size of the log file. This optimizes the initial run since we don't have to read and analyze it, and configures the offset to the proper starting point for subsequent runs.