upstream version auto-detection

This patch implements upstream version detection using two methods:

1. from HTML listing if upstream.archive_url is set (default)
2. using custom upstream.version_script if set

Optional htmllistparse module is required in order for 1) to work.

get-archive was updated to use the new version detection.

This patch also includes related quality of life changes:

• upstream-related config options moved to new [upstream] section
• apkg config is now exposed using Project.config_get() which allows convenient access from code and templates, i.e.: project.config_get('upstream.archive_url')
• Project class has new upstream-related methods
• docs updated to reflect new features and config changes