Fix coding style

Use `goog.isDef(x) ? x : {}` instead of `x || {}`
Upgrade to jsdoc 3.3.
Merge pull request #1964 from pagameba/node-jsdoc

Upgrade to jsdoc 3.3.
Merge pull request #1966 from fredj/coding-style

Fix coding style
Adding a JSDoc configuration for generating metadata on exported symbols

This generates info for all symbols annotated with "@todo api" (when Plovr is replaced, this will become "@api").  Generated metadata is written to stdout.  A separate process will handle the output.
Add a task to generate metadata on exported symbols

This runs JSDoc with the "symbols" config, providing a list of source files that have been changed since the previous run.  The output is used to generate metadata files containing exported symbol info for each of the source files.  A separate task will be run to generate the exports.js file.  These same metadata files will be used by a build tool.
Write a single symbols file
Add extends info to symbol metadata
Rename group to package