Add simple baseline run graphing

Add visual representation of baselines in graphs.  When passing a "baseline.<n>=<run_num>" argument to graph, a horizontal baseline will be drawn on the graph derived from the run passed.  The same test and metric are used for each plot.  The baseline is drawn as a thick darker line, in the same color as the original plot. The intended use is to compare something like trunk to the previous release.  This setting is off by default.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@205063 91177308-0d34-0410-b5e6-96231b3b80d8