add argument to pandoc.render to allow for skipping popup

[classabbyamp]

This kind of ties into #8, but it would be nice to be able to specify a keybind to render without getting the popup window every time.

You can do something like

{
    "key": "f5",
    "command": "pandoc.render",
    "args": {"outputType": "pdf"},
    "when": "editorTextFocus && editorLangId == 'markdown'"
}

in keybindings.json and I believe the value of args will get passed to the command, like this:

-     var disposable = vscode.commands.registerCommand('pandoc.render', () => {
+     var disposable = vscode.commands.registerCommand('pandoc.render', (outputType: string) => {

and then some popup-skipping logic could be added or something.

Not 100% sure the details are right, this is just from my investigations into setting up a way to easily trigger a re-render.