Add ability to generate secrets only for select environments

[rogerluan]

Description

Resolves #31

Testing

To test this branch, modify your Gemfile as:

gem 'arkana', git: 'https://github.com/rogerluan/arkana.git', branch: 'roger/select-environments'

And run bundle install to apply the changes.

Then follow the documentation in the README (you can read the diff in this PR) to generate secrets only for select environments.

[nishchal-v]

@rogerluan We have implemented the environment selection using the flag --include-environments flag, if we can make it shorter for something line --environments, or we can provide a shortcut flag like '-ev'.

Tested the PR for selection of environments and works like a charm ✨

[rogerluan]

Thanks for testing it out @nishchal-v ! 🤗

I've added a new shorthand flag for the --include-environents flag. Simply pass -i 😃

I decided to use --include-environments instead of e.g. --environments just so we can possibly implement --exclude-environments in the future if there's such need, without making it too confusing or needing breaking changes :)

[nishchal-v]

Okay, i understand that, thanks for providing the shorthand flag.

Tested the PR for the latest flag change and work fine, I think we are good to go for this PR from the functional point of view.

[rogerluan]

I'm merging this in :) Let me know if anyone encounter any issues!