Document Loader Options

**Is your feature request related to a problem? Please describe.**
My use case is; a cron job that trains on a git repo. I'm using git commit loader rather than file loader (not entirely sure if this is best for me).

The main thing for me is iteratively adding documents to a vector store, rather than once and done.

**Describe the solution you'd like**
I'd love to filter which commits are loaded, similar to how I can filter for only files of certain extensions. In my specific scenario I'd only like to load commits I've not seen before.

**Describe alternatives you've considered**
A generic filter/lambda type function that allows developers to plug in their own conditions on whether a commit should be loaded.