Maintain and serve a REST API to search PGXN mirrors

Changes for 0.20.0

• Removed the `Capfile` and `eg` directory. Examples for managing PGXN can now be found in the pgxn/pgxn-ops GitHub repository.
• Switched from Text::Markdown to CommonMark for parsing and formatting Markdown files (but not MultiMarkdown files). This allows code fences to work and generates nicer HTML in general, but is stricter about certain things.
• The docs indexer now indexes a distribution's README if it is the only documentation it finds in the distribution (#12).
• The docs indexer now strictly links an extension to the doc file specified via the `docfile` key in its `provides` object, even if it's a README (#10).
• The indexer will now index a testing release if there are no stable releases, and will index an unstable release if there are neither stable nor testing releases (#2).
• Updated the SemVer regex when parsing rsync output to the official version published in https://regex101.com/r/vkijKf/ (#16).
• Fix unzipping of distributions to ensure that all directories are readable and executable but not writeable by all, and that files are only readable by all (#15).
• Dropped support for Perl 5.10 and 5.12.