fizzy-oss Rails Engine option

[harisadam]

This may be an opinionated PR, but it appears to follow the 37signals approach to setting up the project on your own infrastructure.

Similarly to fizzy-sass, this update also uses a dedicated Gemfile (Gemfile.oss) that resolves a user-specific Rails engine name fizzy-oss (my minimal demo implementation: https://github.com/harisadam/fizzy-oss).

This engine-driven approach provides more flexible customization options for users.

If the team agrees this is the right direction, I’m happy to follow up with documentation in the README.