Add an option to create gem for Rails

[juanmanuelramallo]

Why

Given we want to ease the start of the development of a gem, we also want to provide the more appropriate defaults for gems that target Rails applications.

What

Investigate the basic structure of a gem targeting a Rails application.
Add the option to the CLI and the tasks needed to fulfil this.
You can also include a link in the readme if there's a good guide to develop gems for Rails apps.

Notes

https://guides.rubyonrails.org/plugins.html