In `main()`:
```zig
app.route(.GET, "/custom/:id/foo/bar", @import("app/views/custom/foo.zig"), .bar);
```
Routes `GET` request with path (e.g.) `/custom/1234/foo/bar` to `bar()`
defined in `src/app/views/custom/foo.zig`.
Routes with an `:id` segment expect a function with three parameters,
routes without an `:id` segment expect a function with two parameters
(i.e. the same as `get` vs `index`).
