Support wildcard/fallback routes

[enriquecaballero]

I couldn't seem to find a way to implement this and it doesn't seem like there are any examples online: at the moment frontend routers such as React Router aren't compatible with Plumber as there doesn't seem to be a way to create a wildcard/fallback route they require. Usually with other api building frameworks, such as Node.js Express, I usually have a route at the end of my other routes as such:

app.use("/", (req, res) => {
  res.sendFile(path.resolve("./static", "index.html"));
});

Would be essential to support a fallback/wildcard route to ensure frontend router compatibility.

...

(If this is already implemented or can be implemented programmatically, could you please provide an example?)

[schloerke]

This solution would ruin API docs until #882 or #883 is implemented.

---

Very clunky, but there is a work around. (See static file handler for an example.)

You can mount a router at /. This mount should have a filter attached to it to handle all requests that it receives.

## Untested

#' @plumber
function(root) {
  fallback <- plumber::pr()
  fallback$filter("fallback", function(req, res) {
    # Do not call `plumber::forward()`
    # Return fallback info
    res.sendFile(path.resolve("./static", "index.html"));
  })

  root$mount("/", fallback)
}

This should also work if you want to perform this on a sub route.