User interface to insert a thematic break

[dselman]

Markdown supports the concept of a thematic break which markdown-pdf interprets as a page break (--- in markdown). Having a toolbar item to insert a thematic break in the markdown editor would allow users to control where page breaks occur in PDFs that are generated from the contents of the editor.

The thematic break should be rendered in Slate to indicate that a page break will occur. E.g.

---- page break ---- 

Or similar.

In CommonMark this is represented by an org.accordproject.commonmark.ThematicBreak node. See:
https://models.accordproject.org/markdown/commonmark@0.2.0.html

[jolanglinais]

Hacktoberfest:

This issue can provide for multiple pull requests.

[Innovator007]

Can you describe more about this issue @dselman.

[jolanglinais]

I think this is about having a button in the formatting toolbar to insert a horizontal rule into the markdown (---).

[waahhhh]

On it!

[waahhhh]

I have created a draft because I am unsure whether removing the horizontal line (backspace) in the method "handleBackspace" does not work or there is a bug in Slate.

[dselman]

markdown-pdf inserts a page break for thematic breaks in markdown, so this would allow people to have some control over pagination.