Add remark-utf8s and remark-code-screenshot to list of plugins

[Swizec]

Hi 👋

I just created two small plugins for Remark and thought they'd fit your list of existing plugins.

[sobolevn]

@Swizec please, address these issues:

doc/plugins.md
    42:5-42:54  warning  Marker style should be `*`                unordered-list-marker-style  remark-lint
          42:7  warning  Incorrect list-item indent: add 2 spaces  list-item-indent             remark-lint
  180:5-180:66  warning  Marker style should be `*`                unordered-list-marker-style  remark-lint
         180:7  warning  Incorrect list-item indent: add 2 spaces  list-item-indent             remark-lint

[Swizec]

@sobolevn I copied that from the other lines and just verified with GitHub's online editor that it is exactly the same.

I don't know how else to fix it.

[wooorm]

Problem was that you typed them over, and there’s a slight difference between the normal dash (-) that starts a list item and an em-dash (—), used to delimit the plugin from the description, which you don’t really see in monospace.

Thanks for fixing!