Viewed item background highlighting

[AaronDingman]

Summary

Adds a new option to the options page to allow for toggling background highlighting of already viewed files. This allows for additional information at a glance when combined with the checkmark.

The color was selected to be the same as the github additional line added color.

Change Preivew:

Options page addition:

[kopach]

hi @AaronDingman, I was also struggling with the same issue - those checkmarks are not visible enough when there are a long list of files. So, I've implemented this feature to solve the same issue: https://github.com/berzniz/github_pr_tree/pull/180 Please, take a look and let me know what do you think.

[berzniz]

Thanks @AaronDingman & @kopach for the contribution. This will have to be planned for version 2 (which will be based on Manifest 3)