Refactoring

[unguiculus]

This refactors the code to improve testability and so that it can be better integrated into an upcoming GitHub app.

• Rename the binary to cr (similar to ct for chart-testing) and update package structure along with that
• Fix incorrect license headers
• Change default config file locations (analogous to chart-testing)
• Refactor index and release creation and introduce new Releaser struct
• Simplify error handling. In particul, I think we should not try and parse GitHub specific error codes. This doesn't gain us anything. The best we can do is log errors and exit
• No longer check if a release already exists. Just try and create it which will fail if the release exists. So, the result is basically the same with less code
• No longer download chart packages for index creation. This is unnecessary and does not scale
• Remove Chart.yaml from releasse assets. Its unnecessary and make things more complicated
• Encapsulate GitHub API within package without exposing it
• Expect all charts to be released in a specific directory removing the recursive option
• Add unit tests

Fixes: #18

[unguiculus]

Added a first round of tests. More to follow...

[unguiculus]

Updated PR description. Let's move changelog and packaging to a follow-up PR.

[scottrigby]

🤩👏