Add an option to disable automatically adding assets to the asset pipeline

[OddEssay]

Allows projects which do not use the rails asset pipeline, such as those using webpacker to behave more predictably by skipping the initializer step that configures the assets.

Behaves normally without the config option, so existing users won't notice any change in behaviour.

[kitop]

Hi @OddEssay ! Thanks for this! Love it!

Looks like tests are failing due to some bundler incompatibility in newer ruby versions. I'm gonna do a bit of a cleanup in there, so you can rebase this branch and we can merge.

[kitop]

@OddEssay Done! Can you rebase whenever you get a chance? Thanks!