Comicon

- Breaking API changes to allow easier operation with Mandown: `comicon.{BaseMetadata, BaseComic, BaseChapter}` → `comicon.{Metadata, Comic, Chapter}`
- Many things are now imported into the main `comicon` namespace for easier access
- New `SupportedInputList` / `SupportedOutputList` to dynamically determine which formats are allowed

Initial release!

Supported conversions

| Format | Convert from? | Convert to? |
| --- | --- | --- |
| CBZ | :heavy_check_mark: | :heavy_check_mark: |
| EPUB | :heavy_check_mark: | :heavy_check_mark: |
| PDF | :heavy_check_mark: | :heavy_check_mark: |