功能请求：MarkdownPreviewToggle命令

[serend1p1ty]

现在需要定义两个快捷键，一个用来运行MarkdownPreview，一个用来运行MarkdownPreviewStop。
希望有一个MarkdownPreviewToggle命令，这样只需要一个快捷键就行了。

[cabaalexander]

Hello, @ppnman and @iamcco 👋 Rignt now I'm achieving this behavior with function 👇

And I map it to C-p to (p)review (Example below)

@iamcco Maybe I can help with a PR if you like the idea. 🙂

[iamcco]

@cabaalexander

It's OK for me, PR is welcome.

And I map it to C-p to (p)review (Example below)

By the way it will be better if it can let user choose what key to map.

[iamcco]

Thanks to @cabaalexander ，can use nmap <C-p> <Plug>MarkdownPreviewToggle now.