Add more coverage for array markdownExtensions

[ximet]

In a previous issue #10 , you added fix with the array markdownExtensions, but you didn't add full coverage:

Markdown Extensions:

.markdown .mdown .mkdn .md .mkd .mdwn .mdtxt .mdtext .text .Rmd

Examples of extension usage:

GitHub: markdown, mdown, mkdn, mkd, md (source)

Elements: Markdown Editor: markdown, mdown, mdwn, md

Vim markdown: markdown, mdown, mkdn, mdwn, mkd, md

Bitbucket: markdown, mdown, mkdn, mkd, md, text (source)

R Studio: Rmd

[kbrsh]

Sold uses the same ones as GitHub in order to encourage a smaller set of extensions to be used consistently.

const markdownExtensions = ["markdown", "mdown", "mkdn", "mkd", "md"];

I think that's fine as those are the most popular extensions as well.

[ximet]

I do not agree with you.

[kbrsh]

I'm curious—what extension do you use?🤔

[ximet]

I use: mdwn and mdtext in my some projects