Refactor of min/max for subcommand

[henryiii]

The old required_subcommand had several issues. This PR fixes those with a more thoughtful solution. The range of allowed subcommands can now be explicitly set, and CLI11 finally stops trying to match subcommands after the max number allowed is obtained. The changes:

• require_subcommand now has a two-argument form, with min and max. Setting max to 0 means unlimited subcommands.
• The negative value on the single argument form has been changed to the more useful "At most" -N subcommands. So -1 would mean 0 or 1. (Previously: would set 1 or more for any negative argument)
• The error type is now the correct "ExtrasError" (or whatever the final subcommand does) instead of weird RequiredError if you have too many subcommands.
• The maximum number of subcommands is inherited, as well. This is intended to allow someone who prefers -1 to set the default once. It only affects subcommands that have subcommands, obviously.

I've left .require_subcommand() as setting 1 or more, though 1 might be better; I did that because 1 is intuitive for someone requiring 1 subcommand, and 1 or more is not obtainable with a single argument otherwise (and is rather undiscoverable as .require_subcommand(1,0)).

As part of the extended testing, I've reduced the "grabbyness" of the unlimited argument options. They now no longer eat up options if positionals remain (including unlimited positionals).

[codecov]

Codecov Report

Merging #51 into master will decrease coverage by 0.07%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   99.67%   99.59%   -0.08%     
==========================================
  Files           9        9              
  Lines        1241     1249       +8     
  Branches      251      252       +1     
==========================================
+ Hits         1237     1244       +7     
- Misses          4        5       +1

Impacted Files	Coverage Δ
include/CLI/App.hpp	99.28% <93.75%> (-0.14%)	⬇️

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[codecov]

Codecov Report

Merging #51 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage    99.6%   99.76%   +0.16%     
==========================================
  Files           9        9              
  Lines        1256     1289      +33     
  Branches      252      257       +5     
==========================================
+ Hits         1251     1286      +35     
+ Misses          5        3       -2

Impacted Files	Coverage Δ
include/CLI/App.hpp	100% <100%> (+0.28%)	⬆️

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