server: Make it easier to add special handlers.

Currently, this is used for serving static content.
Added support for simple SVG graphs for metrics.

The graph will be displayed when querying a metric. The graph itself mostly
uses default settings of the Plotinum package for now.
graph: Use a date string for x-axis tick markers.

This is a very basic implementation which is not well suited for all time
ranges but it's good enough to get started.