FileRoutes: support for custom directory

[indeyets]

Being able to customize routes directory of <FileRoutes> component would allow to have more flexibility and ability to have several independent routers.

I'm thinking about something like this:

<div id="menu">
  <Routes>
    <FileRoutes directory="src/menu-routes" />
  </Routes>
</div>

<div id="body">
  <Routes>
    <FileRoutes directory="src/routes" />
  </Routes>
</div>