Routing for Hosts and Subdomains

[lovelydinosaur]

• Adds app.host(host, app), which mirrors the existing app.mount(path, app).
• Adds a Host(...) routing class, which mirrors the existing Mount routing class.

Closes #360

Examples:

Website and API, with separate routing rules:

app = Starlette()
site = Router()  # Use eg. `@site.route()` to configure this.
api = Router()  # Use eg. `@api.route()` to configure this.

app.host('www.example.org', site)
app.host('api.example.org', api)

A custom subdomain:

app = Starlette()

site = Router()  # Use eg. `@site.route()` to configure this.
subdomains = Router()  # Use eg. `@subdomains.route()` to configure this.

app.host('www.example.org', site)
app.host('{subdomain}.example.org', api)

Features:

• Subdomain components are available in URL reversing, eg. request.url_for("users_list", subdomain=...)
• Namespaces route naming is supported. eg. app.host('api.example.org', api, name="api') will then require reverse lookups like eg. request.url_for("api:users_list")
• Matched subdomain parts are available on the existing request.path_params.

Considerations:

• The name of request.path_params isn't exactly correct anymore. Should we move to request.route_params or something?
• For testing purposes users will need an easy way to mock up the host for requests, rather than eg. 127.0.0.1. Do we want that available in Starlette or in Uvicorn? Or do we just point folks at how to edit their hosts file?
• We might like subdomain parameters to be optional. If they're not included then request.url_for should default to current domain. (Assuming it matches the pattern?) Actually probably forget about that - we don't expect it with submounted paths, we shouldn't expect it here.
• We need to document this.
• I'm also becoming less convinced with the benefits of the decorator style routing, vs. composing route instances.

[lovelydinosaur]

Let's get this in. We can expand the documentation in due course.