chigger: allow to overwrite previously rendered movie
chigger: allow to overwrite previously rendered movie

Adding option to allow to overwrite a previously rendered movie.
When scripting movie rendering, we may not want to wait for user
to answer the question if the existing movie file should be
overwritten - ffmpeg does that.

This is simply accomplished by passing '-y' flag to ffmpeg which
will cause that all questions asked by ffmepeg will be answered by
'yes'.

Closes #13924

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