Ignore certain columns by default

[fatkodima]

We are currently using paper_trail and have billions of items in the versions table and the table is huge.

One of the reasons is that we have touch: true in associations and every change in the child record touches parent record and a new version record for this is created.
In reality, most (all?) projects do not need to track just this column change, additionally to other columns like lock_version.

So, I think these should be ignored by default. I propose to not change the library to ignore it, but extend the generator to also generate an initializer file for people to use in their projects and to have these columns ignored in it. Having this initializer will be also easier for users to see what can be configured and have a reasonable configuration already in place.

An alternative library (audited) already does this by default https://github.com/collectiveidea/audited/blob/4ffc6c14c4d1692f1ad626f1a4e95d28c968e6bf/lib/audited.rb#L38

Wdyt? I can help with the PR.

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[fatkodima]

Valid. I still want to implement this.

[ngobin]

I have also wanted to create a paper trail initializer file for a while as well, so I took this opportunity to make a PR. The initializer file disables versioning on touch events by default, as suggested (imo a sensible default, we have this in our app).

I also included the other key config options mentioned in the readme, added a test, and updated the readme and changelog. This is my first contribution to paper_trail, please let me know if there is any feedback or comments!