create +global_rails flag to :lang ruby

Rails have a lot of different file types (.rb, .yml, .env, .js, .coffee, etc), and is very pratical to use projectile-rails globally. With this feature, we enable projectile-rails globally.

Today, we can make it using:
```
;; Projectile globally with SPC r
(require 'projectile-rails)
(map! :leader "r" #'projectile-rails-command-map)
```

would be very nice to have this for everyone