Plugin: add attributes support for better highlight

Hi, I create a plugin joplin-plugin-attrs for adding markdown-it-attrs support to joplin. So we can easily add highlight to our notes which can help us quickly catch important part in notes.

For example

This line one{..c-red}
This line two{..c-yellow}
can generate html
<div css-module="c-red">This line one</div>
<div css-module="c-yellow">This line two</div>

I add several build in css, and people can add more style in Custom stylesheet for rendered Markdown setting.


You can also use ==word== to highlight some text. Although your plugin is more flexible since it supports multiple colours.