Allow converting every filetype to markdown by default

[benlubas]

I'd pretty much like to be able to set these options all at the top level and have them work the way they currently do when they're in the custom_lanugage_formatting.python table, just for all languages.

{
    extension = "md",
    style = "markdown",
    force_ft = "markdown",
}

I'm currently not aware of a way to do this

[GCBallesteros]

If I understand correctly you want to be able to configure the extension and force_ft options in the same way you can do with the style right now. That's perfectly reasonable. I will make it happen next week.

[GCBallesteros]

Closed by #11