Swift package update (JohnSundell#68)

Update Publish’s dependencies to their latest versions:

- Ink `0.4.0`
- Plot `0.6.0`
- Sweep `0.4.0`

Also use Sweep’s new `firstSubstring` API.

RSS: Add option to specify title suffixes for items (JohnSundell#52)

This change adds a new `titleSuffix` property to `ItemRSSProperties`
which, just like `titlePrefix`, enables a string to be attached to
an item’s title when it’s being rendered as part of an RSS/podcast feed.

Dependency update (JohnSundell#36)

- Files has been bumped to version `4.1.1`.
- Plot has been bumped to version `0.5.0`.

RSS/Podcast feeds: Invalidate cache when item count changed (JohnSund…

…ell#18)

When new items were added, even if those items don’t have a later
modification date than the previous generated site, always re-generate
all RSS and podcast feeds.

Initial commit