Rename to codeowners, extend configuration capabilities, add pre-commit-hook support

[robertbublik]

Description

This PR changes a few issues at once which I found while getting to know the project. In order to allow the use of codeowners-validator as pre-commit hook a lot of changes were necessary. As described in #140 pre-commit hooks do not really work with env variables and flags are currently not supported by the code base. Additionally in #116 it was considered to rename codeowners-validator to codeowners in order to make the project more future proof for further functions.

Changes proposed in this pull request:

• rename codeowners-validator to codeownersand create a validate subcommand which inhibits the current capabilities of this project
• replace envconfig with cobra and viper
• add support for flags and config files. environment variables require the prefix CODEOWNERS_.
• place api and config into own packages
• add pre-commit-hooks.yaml file

Related issue(s)

• Release v1 #116
• Add native support for pre-commit hook #140

Additional information

I fixed most of the integration tests but can't run the ones which require an API key. The other tests run fine.

One issue I wasn't able to fix is in the commit c372b63. I have no idea how to add a custom type as flag since I need to convert it somewhere. I would appreciate help there :) Other than that it's very possible I made some errors or mistakes which could be solved more elegantly. Feedback is very welcome!

This PR also includes #197.

[robertbublik]

I tried to rebase and I ran into a lot of conflicts. I can fix them but first I wanted to ask, if you'd prefer to make this PR perhaps a little smaller? I can drop the renaming to codeowners if you prefer. Also I'd still have to take a look how to accept custom types as a flag. 🙂