Create unmarshall-feed.js

Part of plucking feed 'parsing' into its own file
Modularize opml.js

Refactored as module, avoid exporting of private helper functions,
inline some minor functions, reduce LOC and some unnecessary bloat
Cleanup OPML module

Cleanup comments, delete extra lines, misc conventional changes, inline
certain functions that are only called from one place when it does not
reduce clarity