SPA routing

I'm setting up an SPA application using Vue Router with the history mode, so I'd like to redirect all non-static-file requests to the static index.html file served by `app.HandleDir("/", "./frontend/dist")`.

I've tried the following:

```go
app.Get("/*", func(ctx iris.Context) {
ctx.View("index.html")
})
```

But this overrides the static file routes and other API routes, so what's the way to handle all other routes?

Start by reading the routing behavior around app.HandleDir("/", "./frontend/dist"), app.Get("/*", ...), and ctx.View("index.html"). Determine how a fallback for non-static-file requests can coexist with static file and API routes; done means SPA history-mode URLs serve index.html without overriding those existing routes.

Written by the indexing model from the issue text.