Add a mtime style flag to filter out log files based on modification time.

[asachs01]

In internal ref https://secure.helpscout.net/conversation/1825973406/28629?folderId=5845954, it was mentioned that there is a feature in Tivoli's log check that allows users to filter out logs based on the modification time. The user provided the following help text to illustrate the features that the Tivoli log check provides.

FileComparisonMode
Specifies which log files are monitored when more than one matches a wildcard pattern. The
following values are available:
CompareByAllMatches
This value is the default behavior. All files that match the wildcard pattern that is
specified in LogSources are monitored.
CompareByLastUpdate
Of the files that match the wildcard pattern that is specified in LogSources, the file with
the most recent last update timestamp is monitored.
CompareBySize
Of the two or more files that match the file name pattern criteria, the bigger file is
selected for monitoring. Do not use CompareBySize with multiple matching files that are
being updated at the same time and increasing their file sizes. If the largest file is subject
to frequent change, monitoring might continually restart at the beginning of the newly
selected file. Instead, use CompareBySize when there is a set of matching files, but only
one is active and being updated at any specific time.
CompareByCreationTime
Of the files that match the wildcard

[asachs01]

@portertech @fguimond we've got TD bugging us about this again. Can we bump this up and maybe start some work on getting an mtime-like implementation in the plugin?

[fguimond]

@jhenderson-pro to circle back with the customer to clarify the requirements.