Add setup option for custom markdown filetypes

[ddxtanx]

For literate markdown files, or generally any kind of markdown file where users might want to include other filetypes, the filetype reported by vim might not literally be "markdown" but something like e.g. "rzk.markdown" for literate rzk files. To remedy this I introduced a setup option ft to allow users to specify which filetypes they would like to consider as markdown files. Hopefully this gels well with your intended style for this plugin but please let me know if you have any qualms about this implementation.