Store the feature's commonly used id

To not clobber the feature's attributes, this is a separate
member property.
Store the KML Placemark identifier
Merge pull request #2 from fredj/fid

Store the KML Placemark identifier
Merge pull request #733 from ahocevar/fid

Store the feature's commonly used id. r=@fredj,@bartvde
Add parser.jsdoc file for docs on parser options.

Pointer by @ahocevar in #735
Merge pull request #736 from bartvde/jsdoc2

Add parser.jsdoc file for docs on parser options (p=@ahocevar, r=@bartvde)