Mechanism for passing arbitrary extra config to formatters

Lots of formatters have extra config users may want to pass, and creating a top-level flag for each one will get cumbersome. We should consider a generic mechanism for passing flags through to formatters.

Factors to consider:
- These should be attached to the fine-grained formatter name, not the language, because we don't want to accidentally pass them to a different formatter that doesn't support them when the user switches formatter.
- Some might conflict with default flags. For instance, a user might want to pass `--type` to js-beautify and find that it conflicts with the `--type` flag we already pass by default.
- Most config would be per-path rather than global preferences. For instance, a user will want to configure an `--aosp` flag when working on Android Java, but then be surprised if it's applied to other Java code they're working on later.
- If possible, these config options should live in some project-local config file like `.clang-format` or `.editorconfig`, not something codefmt-specific, and codefmt should just try to be smart about detecting and not conflicting with such config where appropriate.

この issue では、プロジェクトローカルな設定ソースの候補として .clang-format と .editorconfig を挙げ、例として js-beautify の --type と --aosp を示している。まず、formatter 固有のオプションが現在どのように選択されているかを整理する。完了の定義は、競合する defaults と意図しない伝播を避ける、formatter ごと・パスごとに決定された仕組みがあることとする。

索引モデルが issue の本文から書いたものです。