benchmarks: draw relative change references in trends plots
benchmarks: draw relative change references in trends plots

Draw and label +/- 1% and 5% lines in benchmark trend plots - to help
give a better feel for how much things are/aren't changing.

ref #9834

bwspenc/moose,milljm/moose,andrsd/moose,sapitts/moose,permcody/moose,SudiptaBiswas/moose,bwspenc/moose,laagesen/moose,dschwen/moose,nuclear-wizard/moose,nuclear-wizard/moose,milljm/moose,permcody/moose,harterj/moose,idaholab/moose,milljm/moose,SudiptaBiswas/moose,friedmud/moose,friedmud/moose,lindsayad/moose,dschwen/moose,andrsd/moose,laagesen/moose,andrsd/moose,jessecarterMOOSE/moose,harterj/moose,YaqiWang/moose,dschwen/moose,jessecarterMOOSE/moose,SudiptaBiswas/moose,laagesen/moose,lindsayad/moose,sapitts/moose,sapitts/moose,harterj/moose,nuclear-wizard/moose,andrsd/moose,dschwen/moose,YaqiWang/moose,nuclear-wizard/moose,SudiptaBiswas/moose,idaholab/moose,dschwen/moose,bwspenc/moose,YaqiWang/moose,friedmud/moose,andrsd/moose,friedmud/moose,milljm/moose,SudiptaBiswas/moose,jessecarterMOOSE/moose,bwspenc/moose,sapitts/moose,lindsayad/moose,harterj/moose,YaqiWang/moose,jessecarterMOOSE/moose,permcody/moose,lindsayad/moose,laagesen/moose,idaholab/moose,sapitts/moose,bwspenc/moose,harterj/moose,milljm/moose,permcody/moose,lindsayad/moose,jessecarterMOOSE/moose,idaholab/moose,laagesen/moose,idaholab/moose