Improve developer flags

@devoncarew

* Make adding new flags to SparkFlags even easier
* Sequentially read and set flags from:
  * `<app>/app.json`
  * `<app>/user.json`
  * `<project_location>/.spark.json`
  
  Each of these overrides flags set in the prior ones. `user.json` is mostly for setting `grind deploy`-resilient flags in the working copy of Spark. `<project_location>/.spark.json` is mostly to allow setting flags for a packaged copy of Spark downloaded from CWS (when bootstrapping Spark from Spark or debugging user-reported bugs).