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.
Adds reference to SLUGIFY_SOURCE

When working on my site, I wanted SLUGIFY_SOURCE, but I came across PATH_METADATA first and enabled that for a while. Better to include the cross-reference where people can find it.
Fixes #1420: Handle multiple definitions of standard metadata for Markdown
Merge pull request #1445 from avaris/fix_1420

Fixes #1420: Handle multiple definitions of standard metadata for Markdown
Merge pull request #1438 from mlissner/patch-2

Add reference to SLUGIFY_SOURCE
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