Allow Markdown in separate md file

One idea I've had when creating slides with Markdown is to allow to put the Markdown text into a separate file. The impress html presentation would reference the file and its contents would be added as text content for the element. Something like:

Start with impress.js presentation initialization and the HTML element using data-markdown-file="content.md"; inspect how Markdown content is currently handled. Define the loading and rendering path for the referenced file, then verify that its Markdown appears as the element's text in an HTML presentation.

Written by the indexing model from the issue text.