Allow iframes for all of PromDash.

We want to be able to show any PromDash content in iframes, so this
removes the X-Frame-Options for all views.
Merge pull request #173 from prometheus/allow-iframes

Allow iframes for all of PromDash.
Update README.md
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