Update browser with contents from current buffer

Something like `:MarkdownPreviewUpdate` could render contents from current buffer into existing browser. This would allow to have support for editing many Markdown files, but rendering everything into single browser.

Similar plugin [vim-markdown-composer](https://github.com/euclio/vim-markdown-composer) has something like what I'm describing called `:ComposerUpdate`. Unfortunately it doesn't have synchronised scrolling which is a killer feature for me.