promote variables to constants if they are the same in a graph

* more concise display
* allows to later apply settings on graphs more easily
add metric inspection page
make display of graph title, target name more clear

deterministically hash each tag key (server, type, ..) into a
html color code, and display tags in a label of that color,
so that tags can be more easily recognized

graph name:
* all constant and promoted constant tags as colored labels
* 'what', 'type', 'target_type' first; 'server', 'plugin' last
* (or the graph key for constant-less graphs (predefined graphs))

target name:
* all variables as colored labels or 'single_target' if none.
* link to metric inspection page (adds graphite_metric to target)
* (or the name if set (predefined graphs))
document first steps
deprecate default_graph_options; introduce preferences; auto-vtitles

it's not the task of a plugin to control how a graph is rendered;
remove the concept of default_graph_options, and introduce these
features that make up for it:

* automatically set a clear, consistent vtitle based on
  what/type/target_type tags
* implement a preferences mechanism where you can match on graph
  properties and modify the configuration accordingly
fix some target_types