Add configuration for limiting old revisions count.

[jung-hunsoo]

I'm about to implement this package to my webapp.

And it'd be great to configure the maximum DB remaining revisions. Generally there're some use cases in revisions; track full history or compare/revert changes. And the latter commonly don't need to store all the changes.

It can be limited per model(table) or total revisions count. Both will work on FIFO.

[Qqwy]

This is an interesting proposal. Revisionair was first and foremost created to create a full 'audit trail' of your data structures, but I can see this feature being useful in certain applications.

I have no idea what the API of this should look like, though. Currently, Revisionair is functions-only, without any central configuration (and this has many upsides; the most important being that it can be used in many different places in different ways within the same application).
Any suggestions?

Currently I think adding a delete_versions_older_than(my_structure, n_versions) (the arguments being configurable in the same way as delete_all_versions_of, although another approach could be to add an extra option to the options-keywordlist that store_revision accepts.