Release v0.19.8

[johanjanssens]

1. Automatic link prefetching #587

Implemented automatic link prefetching. The link prefetcher is now automatically injected in the page if following conditions apply:

• The page format is html
• The page is cacheable
• Static caching is enabled

2. Sync with changes in framework 3.4.13 #597

Synced code with changes made in joomlatools/joomlatools-framework#468 and joomlatools/joomlatools-framework#470

3. Improve metadata handling #593

• Opengraph metadata is not added to a page if og:type is null or false
• The robots value doesn't need to be an array, a single value string is now also accepted eg `robots: none``

4. Implement page singleton #591

Fixes

• Check if path exists before calculating hash Check if path exists before calculating hash #595
• Handle http cache parse errors gracefully Handle http cache parse errors gracefully #589