Fixes #1395: extends pelican.utils.strftime with `-` prefix to strip leading zeros

Adds the ability to use `-` prefix with C89 format codes to strip any
leading zeros.
Add feed_written signal
Merge pull request #1446 from avaris/enhanced_strftime

Fixes #1395: extends pelican.utils.strftime with `-` prefix to strip leading zeros
Merge pull request #1408 from arty-name/feed-written

Add feed_written signal