Add option to pre-filter the results returned from S3

[ianbamforth]

I'm checking file age on a bucket containing over 24000 items, and it's really slow. Would be useful to be able to supply a prefix or filter in order to narrow down the results that S3 returns, before the checks are performed on --bucketfolder.

[matt448]

I think the pull request I just merged should take care of this. If not I will re-open this issue.