Config option 'prefer-source' added to schema, refs #553
Support for 'prefer-source' config setting, refs #553
Install/update now uses the new config variable 'prefer-source', fixes #553
Minor code reorg to reduce duplication
Scratch 'prefer-source'; 'preferred-install' is the bee's knees
Merge pull request #1658 from miklosm/iss553

Adds 'prefer-source' option to config