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.
* youtube video url
* WIP, channel /user/ and /c/ should be valid
* unit tests passing
* soundcloud artist url
* soundcloud username
* break configs with name change
* Update all yamls with urls
* update all docs
* more negative url tests
