Add filter to $is_single

[ptrsmk]

This allows for third party code to customize who counts. Example, easily exclude admins from being counted.

[cabrerahector]

Thank you for contributing!

However, I already have this feature planned (I know, it's been sitting there for an incredible amount of time) and will be implementing it in a different way.

[ptrsmk]

Understandable, and I look forward to the feature. However, even if the plugin settings options become more granular, a filter for developers and edge cases is valuable.