Adds use_file_dates option for non git use

[Dschogo]

This PR adds a new option, use_file_dates. When set to true, this option allows the plugin to use the file creation or modification date as the timestamps instead of retrieving it from the git log.

Useful for non git projects

[timvink]

Hi @Dschogo ,

Appreciate the effort you put in!

In this case it would have been better if you had discussed the change first:

https://github.com/timvink/mkdocs-git-revision-date-localized-plugin/blob/af27290b2d88a309903b786e60c0aba0e7cae674/CONTRIBUTING.md?plain=1#L5

I don't think we should add this option:

• More options add cognitive overhead for users; it makes it harder to use this package
• The scope of this plugin is for project that use git (it's in the name of the package)
• If you do use git, the creation/modification dates on the filesystem cannot be trusted as it depends on the git clone date.

I would recommend forking this project, and removing all of the git related code, and releasing a new plugin. It's MIT licensed, so not a problem.

So, closing this PR.