feature request: Add ability for per project ecosystem exemption

[zkoppert]

          Thank you for adding this feature.

I briefly looked through the changes, and it's not apparent to me how a project can mark that a ecosystem shouldn't be added to the dependabot-file. You can exclude ecosystems globally through the exempt_repos config, but can it be done per project?

Originally posted by @vidwah-nte in #140 (comment)

In order to add a per repository ecosystem exemption, we would need to modify the env handling to accept a new variable repo_specific_exemptions that contained a string of repository names and ecosystem exemptions where multiple exemptions per repository are allowed. ie. repo_specific_exemptions = "org1/repo1:docker,github-actions;org1/repo2:pip would set exempt_ecosystems for org1/repo1 to be ['docker', 'github-actions'], and for org1/repo2 it would be ['pip'], while for every other repo evaluated, it would be set by the env variable EXEMPT_ECOSYSTEMS.