Third stage callback

[phlptp]

If merged this PR will add a third callback to apps.

Based on the discussion from gitter

It adds two new function final_callback and parse_complete_callback The `callback function will populate one of these depending on the status of immediate_callaback. immediate_callback will also toggle between them if one is empty. The default is routing to final_callback which has the same behavior as prior to the PR.

Some thoughts about deprecating the immediate_callback but losing it would remove capabilities at present, so I don't think it should be deprecated yet at least until the callback function itself is deprecated.

[phlptp]

I want to see where additional test coverage is needed for this PR, so I think it ready for review, but I will probably need to add some more tests yet.

[codecov]

Codecov Report

Merging #313 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #313   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        3017   3027   +10     
=====================================
+ Hits         3017   3027   +10

Impacted Files	Coverage Δ
include/CLI/Option.hpp	100% <100%> (ø)	⬆️
include/CLI/App.hpp	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #313 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #313   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        3028   3038   +10     
=====================================
+ Hits         3028   3038   +10

Impacted Files	Coverage Δ
include/CLI/Option.hpp	100% <100%> (ø)	⬆️
include/CLI/App.hpp	100% <100%> (ø)	⬆️
include/CLI/ConfigFwd.hpp	100% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fc6e1c7...d36dfe9. Read the comment docs.

[phlptp]

I think everything is ready to go here. I left immediate_callback operation in place to avoid any breaking changes. So callback should behave as before even though the operation is technically redundant. immediate_callback still functions from a user perspective as before even though the actual operations it does are very different.

[henryiii]

I think this would really benefit from having the tutorial book updated. I'll try to merge that into CLI11 in the next week or two.

[phlptp]

Keeping the book up to date will probably be a lot easier once it is merged. I know I would be more inclined to update that if it was in a single repository.