[FEATURE REQUEST] SPA SubPath are not supported

[aveyuan]

Is your feature request related to a problem? Please describe.
SPA SubPath are not supported

it's ok

app.HandleDir("/", iris.Dir("./public"), iris.DirOptions{
  IndexName: "index.html",
  SPA:       true,
})

not ok

app.HandleDir("/admin", iris.Dir("./public"), iris.DirOptions{
  IndexName: "index.html",
  SPA:       true,
})

the index.html static resouce not found

Describe the solution you'd like
support subpath SPA

Describe alternatives you've considered

Additional context

[aveyuan]

should add root in iris.DirOptions