add annotation to support default drilldown

[jspeis]

We will want to add a new annotation called ui_default_drilldown. The purpose of this annotation is help the vizbuilder provide more useful default visualizations, for instance measures like Property value by bucket would be more intuitive by adding an annotation of ui_default_drilldown with a value of Value Bucket that tells the vizbuilder it should default to drilldown on the Value Bucket so that a user would see https://saguaro.datausa.io/visualize?groups=0-Z1sykUT&groups=1-26IlCY&measure=ZX51uM instead of just https://saguaro.datausa.io/visualize?groups=0-Z1sykUT&measure=ZX51uM by default because the latter doesn't really have much useful information because its not broken out by bucket.

I will hack in a prototype in ulysses branch of datausa-mondrian. Once that's up and running it would be great if you can migrate those annotations to the ETL code so that it will persist when the schemas are regenerated.

[jspeis]

this is now hacked into the schema so the next step will be for @hwchen to integrate these annotations into the ETL for future updates