Add support for pruning based on file age

[atc0005]

Right now we're keeping X number of files and relying on filename patterns to help limit the files that are removed. To make this tool more useful, it will need to support age-based matching as well. Other criteria can be added later (e.g., size), but this one seems to be the more useful additoin to include first.

Match age in minutes, hours, days, ... ?

[atc0005]

Scratch notes.

Get current time, calculate based on provided flags, options. This will give us a time value. Then as we crawl Path, we will get modification time of files. If modTime is greater than calculated time, the file is a candidate.

start := time.Now()
... operation that takes 20 milliseconds ...
t := time.Now()
elapsed := t.Sub(start)

• https://golang.org/pkg/time/
• https://golangcode.com/get-unix-time/

[atc0005]

I think I have it, but may have additional polish to apply before I'm done with it.

[atc0005]

The logic for keeping X many files needs to be reviewed. It made sense to have the prior behavior when we were not processing based on modification time (or even considering it at the code level), but now with a set of files of (intentionally) mixed age, the "keep X" logic is being applied to the matches, even when a large number of remaining files are in place.

Example:

ubuntu@testbox:/tmp$ ./elbow --path /tmp --pattern "reach-masterqa-" --keep 1 --recurse --keep-old --ignore-errors --age 120 --extension ".tmp" --extension ".war" --log-level info --log-format text --remove
INFO[0000] Starting evaluation of path                   extensions="[.tmp .war]" file_age=120 file_pattern=reach-masterqa- path=/tmp
INFO[0000] 12 files eligible for removal
INFO[0000] 1 files to keep as requested                  keep_oldest=true
INFO[0000] Ignoring file removal errors: true
INFO[0000] Removing filereach-masterqa-d9db6e2-20190501-1037.war  file=reach-masterqa-d9db6e2-20190501-1037.war removal_enabled=true
INFO[0000] Removing filereach-masterqa-1e14b45-20190507-1210.war  file=reach-masterqa-1e14b45-20190507-1210.war removal_enabled=true
INFO[0000] Removing filereach-masterqa-6e83c12-20190507-1239.war  file=reach-masterqa-6e83c12-20190507-1239.war removal_enabled=true
INFO[0000] Removing filereach-masterqa-a9ea1d0-20190508-1424.war  file=reach-masterqa-a9ea1d0-20190508-1424.war removal_enabled=true
INFO[0000] Removing filereach-masterqa-82a921c-20190510-1334.war  file=reach-masterqa-82a921c-20190510-1334.war removal_enabled=true
INFO[0000] Removing filereach-masterqa-a9ea1d0-20190510-1054.war  file=reach-masterqa-a9ea1d0-20190510-1054.war removal_enabled=true
INFO[0000] Removing filereach-masterqa-a9ea1d0-20190510-1321.war  file=reach-masterqa-a9ea1d0-20190510-1321.war removal_enabled=true
INFO[0000] Removing filereach-masterqa-a9ea1d0-20190510-1326.war  file=reach-masterqa-a9ea1d0-20190510-1326.war removal_enabled=true
INFO[0000] Removing filereach-masterqa-a9ea1d0-20190510-1316.war  file=reach-masterqa-a9ea1d0-20190510-1316.war removal_enabled=true
INFO[0000] Removing filereach-masterqa-c765a45-20190528-1255.war  file=reach-masterqa-c765a45-20190528-1255.war removal_enabled=true
INFO[0000] Removing filereach-masterqa-b1e3655-20190530-1021.war  file=reach-masterqa-b1e3655-20190530-1021.war removal_enabled=true
INFO[0000] 11 files successfully removed
INFO[0000] reach-masterqa-d9db6e2-20190501-1037.war      failed_removal=false
INFO[0000] reach-masterqa-1e14b45-20190507-1210.war      failed_removal=false
INFO[0000] reach-masterqa-6e83c12-20190507-1239.war      failed_removal=false
INFO[0000] reach-masterqa-a9ea1d0-20190508-1424.war      failed_removal=false
INFO[0000] reach-masterqa-82a921c-20190510-1334.war      failed_removal=false
INFO[0000] reach-masterqa-a9ea1d0-20190510-1054.war      failed_removal=false
INFO[0000] reach-masterqa-a9ea1d0-20190510-1321.war      failed_removal=false
INFO[0000] reach-masterqa-a9ea1d0-20190510-1326.war      failed_removal=false
INFO[0000] reach-masterqa-a9ea1d0-20190510-1316.war      failed_removal=false
INFO[0000] reach-masterqa-c765a45-20190528-1255.war      failed_removal=false
INFO[0000] reach-masterqa-b1e3655-20190530-1021.war      failed_removal=false
INFO[0000] 0 files failed to remove
INFO[0000] Elbow successfully completed.

These lines in particular emphasize my point:

INFO[0000] 12 files eligible for removal
INFO[0000] 1 files to keep as requested                  keep_oldest=true
INFO[0000] 11 files successfully removed

Since there were a larger number of files still remaining, shouldn't the "keep X" logic have been satisfied by their presence? Or, perhaps the current logic is best as it applies evenly across all matches?