Add support for inline footnotes

Normally the `remark-gfm` supports the footnotes well; **but it does not support for inline footnotes.**

Need a support for inline footnotes like:

```markdown
Sample for inline footnote^[Text of inline footnote] definition.
```

References:
https://markdown-it.github.io/ (just for example)
https://pandoc.org/MANUAL.html#extension-inline_notes