[FEATURE] Add output_options.keep_max_files to limit number of downloads per subscription

[jmbannon]

Adds a new mechanism to limit subscriptions' number of videos to an explicit number (only supported date ranges prior to this).

Example usage:

output_options:
  keep_max_files: 10

will only keep 10 videos at max.

In addition:

• keep_max_files is bundled into the Only Recent preset and can be used via setting the only_recent_max_files override variable (see examples or README for usage).
• Changed the date_range variable name to only_recent_date_range for more clarity. Usage of date_range will continue to work.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (52d1a58) 96.81% compared to head (240fb7e) 96.78%.

Files	Patch %	Lines
...ytdl_sub/validators/string_formatter_validators.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #820      +/-   ##
==========================================
- Coverage   96.81%   96.78%   -0.03%     
==========================================
  Files          79       79              
  Lines        5114     5140      +26     
==========================================
+ Hits         4951     4975      +24     
- Misses        163      165       +2     

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