Replace in-tree engine_assets with its successor: isolate_assets gem

[botandrose]

Hey Igor,

I'm working on a rails engine of my own: https://github.com/botandrose/trainspotter , and ran into that same problem where I wanted to have my own image and javascript and css assets, but can't rely on the host app's asset pipeline.

So rather than just copy and paste the engine_assets code from rails_performance, I took the time to extract it to its own gem, and polish it up a bit. The result is https://github.com/botandrose/isolate_assets !

Its a bit more capable than engine_assets, and has some nice added features, like being able to use the app/assets directory. Plus the API is really nice, just isolate_assets in the engine definition.

I used this branch to work through the extraction, so I'm pushing it up here as PR, if you don't mind adding a gem dependency to the project. If you'd rather keep it in-repo, that's fine too.

[botandrose]

Hmm, looks like I should have removed the lockfiles from the repo for the CI PR, otherwise the gem cache gets mad. Just fixed in a commit. This should be cherry-picked to master, if this branch doesnt get merged. I can move out to its own PR, too, if you'd prefer.

[igorkasyanchuk]

great gems
wish you to receive 1000 stars

PS: don't forget to promote your gems on some forums, newsletters, etc. You need to do it, otherwise nobody will find them.

You can add a link to isolate_assets in readme of this gem, mentioning how assets work somewhere in the docs.