Support wiki-based repository index

New Plugins:
- bpkg-update: sync repo listings locally
- bpkg-list: list the repo names, with -d flag for details
- bpkg-utils: source-only shared utilities for bpkg plugins

Change-Id: I9ef04a86efa35c61cabd7c4ecfa7c7a134b41c6c
Merge pull request #24 from ccarpita/new/repo-update-list

Support wiki-based repository index