Parsing code from URL (Feature)

If a code is already hosted on github (or elsewhere), you might want to avoid code duplication and parse the code from the url (e.g raw github, gists, ... etc).

For example
```
[import:"marker0"](https://raw.githubusercontent.com/azu/gitbook-plugin-include-codeblock/master/example/src/marker.ts)
```

Start by locating the plugin entry point that handles local code includes; the issue names raw GitHub URLs and gists as desired sources but no files or tests. Review existing include behavior, then determine supported URL forms and verify that hosted code can be included without duplicating it locally.

索引モデルが issue の本文から書いたものです。