Add whitelist option to audit columns

[aszenz]

Add option include_columns to specify the columns to be used for auditing an entity. (only those columns should be audited then, changes in all the rest should be ignored)
Right now one has to ignore a lot of columns per entity to achieve the same thing.

In my opinion, most entities only have a few columns that are relevant for auditing, so it makes sense to just specify those and ignore the rest.

[aszenz]

Maybe this should go into https://github.com/DamienHarper/auditor-bundle, let me know

[DamienHarper]

@aszenz thanks for the feedback, it makes sense. I need to think about it a bit more then I'll eventually add it to the roadmap.