Run code using current file name #183

runSource and knitRmd wrap the current file’s path in some R code and send that code to the terminal.

We could generalise this to a function wrapPath (or something) that has as parameters the R function name and any additional parameters to pass to that R function. Both runSource and knitRmd could be refactored to use wrapPath.

The key benefit of this is that then users could add their own similar commands in their settings.json file, without needing vscode-R itself to be changed.