Support config filename/filestem only

[mwestphal]

--config now support filename/fileStem , which is great and lets us specify a config file for thumbnails.

• Add support for filename/fileStem in --config
• Update/Improve doc
• Fix an issue with options not read from config file, introduced in #396
• Add a thumbnail.json and use it in thumbnailers
• Add tests

[codecov[bot]]

Codecov Report

Merging #399 (b663371) into master (baca08d) will increase coverage by 0.03%. The diff coverage is 95.00%.

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
+ Coverage   94.90%   94.94%   +0.03%     
==========================================
  Files          91       91              
  Lines        4553     4567      +14     
==========================================
+ Hits         4321     4336      +15     
+ Misses        232      231       -1     

Impacted Files	Coverage Δ
application/F3DOptionsParser.h	100.00% <ø> (ø)
application/F3DConfigFileTools.cxx	78.04% <60.00%> (+1.30%)	:arrow_up:
application/F3DOptionsParser.cxx	97.93% <100.00%> (+0.08%)	:arrow_up:
application/F3DStarter.cxx	98.05% <100.00%> (+0.05%)	:arrow_up:

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[mwestphal]

last push is only commit reorg.