Add support for non-root applications (base HREF?)

[cedx]

This library assumes that the application will be mounted at the web root. But in real life, this is not always the case. When the application is mounted under a non-root path, we must do some hacks like that to fix the route/location (in this snippet, the mount path of the application is provided by the <base href="..."> tag):

import js.Browser.document;
import tink.Url;

using haxe.io.Path;

new BrowserRouter<Route>({
    routeToLocation: route -> {
        final baseHref = document.head.querySelector("base").getAttribute("href").removeTrailingSlashes();

        // => Need to add the mount path to every URLs!
        switch route {
            case Homepage: '$baseHref/';
            case Foo: '$baseHref/foo';
            case Bar(baz): '$baseHref/bar/$baz';
            default: '$baseHref/error/404';
        }
    },
    locationToRoute: url -> {
        final pathParts = url.path.parts().toStringArray();

        // => Need to strip the mount path from every URLs!
        switch pathParts.slice(Url.parse(document.baseURI).path.parts().length) {
            case []: Homepage;
            case ["foo"]: Foo;
            case ["bar", baz]: Bar(baz);
            default: ErrorHandler(404);
        }
    }
});

It would be cool if this library could handle the mount path by itself, by reading the <base href> tag or via a dedicated property (let's name it baseHref for now):

new BrowserRouter<Route>({
    baseHref: "/apps/my_app",
    routeToLocation: (...),
    locationToRoute: (...)
});