Namespace template vars as var[key]=val in the URL.

This opens up the possibility of using other hashbang query string
variables for purposes other than setting template variables (initial
fullscreen mode, initial time range settings, etc.).
Allow starting a dashboard in fullscreen mode via URL parameters.

https://github.com/prometheus/promdash/issues/127
Merge pull request #174 from prometheus/namespace-user-vars

Namespace user vars and allow starting dashboards in fullscreen mode