Add option for preventing rails credentials editing without environment

[marcqualie]

The rails default is to use master.key for development, then require -e for all other environments. This is odd default behaviour, but likely for backwards compatibility.

Prevent just doing rails credentials:edit and required the -e development to explicitness.

[marcqualie]

It seems this will be pretty difficult to do without PR'ing against rails core, or overriding the rake commands that are defined

There are a couple of hard coded issues:

default_environment: nil

https://github.com/rails/rails/blob/master/railties/lib/rails/commands/credentials/credentials_command.rb#L28

This prevents diffcrypt from manually sending in the environment.

options[:environment]

https://github.com/rails/rails/blob/master/railties/lib/rails/commands/credentials/credentials_command.rb#L82

Rails assumes that if options[:environment] is not set, then the user should work with the config/credentials.yml.enc file and config/master.key