Introduce multiple GitHub token support

[lavoiesl]

This enable multiple GitHub token support.

It starts by looking at whether there is a repo-specific token, then fall backs to more generic ones:

• SMART_TODO_GITHUB_TOKEN__<ORG>__<REPO>
• SMART_TODO_GITHUB_TOKEN__<ORG>
• SMART_TODO_GITHUB_TOKEN (Default)

The <ORG> and <REPO> parts should be uppercased and use underscores.
For example, Shopify/my-repo would become SMART_TODO_GITHUB_TOKEN__SHOPIFY__MY_REPO=....

This is particularly helpful when needing to access multiple private orgs, each having their own private GitHub app

[etiennebarrie]

The <ORG> and <REPO> parts should be uppercased and use underscores. For example, Shopify/my-repo would become SMART_TODO_GITHUB_TOKEN__SHOPIFY__MY_REPO=....

I wonder why we do that. There's no limitation for environment variables names beyond not containing \0 or =. I guess shells don't make it super easy to use environment variables with slashes, but with /usr/bin/env or using Kubernetes I don't think it would cause any issue? e.g.

env SMART_TODO_GITHUB_TOKEN__shopify/my_repo=42 ruby -e 'p ENV.select { /FOO/ =~ _1 }`
# {"FOO/BAR"=>"42"}

[lavoiesl]

TIL.

But while that might be true, it feels brittle, unconventional, and a bit jarring. I went for this design to adhere to what I feel is an implicit standard.

For example, while this may work:

$ env SMART_TODO_GITHUB_TOKEN__shopify/my_repo=42 ruby -e 'p ENV.select { /repo/ =~ _1 }'
{"SMART_TODO_GITHUB_TOKEN__shopify/my_repo"=>"42"}

This doesn't:

$ SMART_TODO_GITHUB_TOKEN__shopify/my_repo=42 ruby -e 'p ENV.select { /repo/ =~ _1 }'
zsh: no such file or directory: SMART_TODO_GITHUB_TOKEN__shopify/my_repo=42

But this does:

$ SMART_TODO_GITHUB_TOKEN__SHOPIFY__MY_REPO=42 ruby -e 'p ENV.select { /MY_REPO/ =~ _1 }'
{"SMART_TODO_GITHUB_TOKEN__SHOPIFY__MY_REPO"=>"42"}