Spiders-for-all

Docs updated

Bug fix:

- Use the first audio of bilibili-api instead of merging all audios to one

Bug fix:

- Fix downloader's state when unexpected exception occured

Optimization:

- Optimize the behaviors of some cli entries

**Breaking changes**:

- Use `HttpClient` to send http request instead of using requests directly, that will retry automatically if the request failed or something - wrong happened when trying to get items from a response with code 200.
- More reasonable module design. Now you should use `python -m spiders_for_all PLATFORM SUB_COMMANDS` to execute some command instead of `python -m spiders_for_all.{platform}`.
- Platform `xhs` now is supported, which contains function like `download-by-note-id` and `download-by-author`.

This is a hotfix release for issue 8

New features:

- Download videos from bilibili by the author mid

Bugs fixed:

- Downloader does't exit if one of the liner tasks failed