Second iteration: add support for dynamic routes

[essenciary]

This is a bit more complex, for routes of form /a/:param/b where :param is dynamically passed and multiple URLs are generated for the single route.

The :param argument should be a iterable or a function and GenieBuilder.build should generate all the different pages for the route, corresponding to all the elements.

Architectural questions:

1/ can we handle one param and multiple params at once? That would be ideal. Maybe if we define some sort of interface and only use functions which must return a NamedTuple of :params at each iteration.
Such routes look like: /a/:param1/b/:param2
-> the challenge here is the combinations of param1 and 2 - ex if we have users and orders, we'd have users from id A to id Z, and for each user we'd have orders from id A to id Z.

2/ how do we define these rules? We could use a static.jl file where for each route we define the iterator -- but this would duplicate the routes definitions. I think the best would be for GenieBuilder to specialize Genie.Router.route to accept an extra argument, a lambda for the iterator rule.

So basically the routes.jl file would look like this:

using GenieStatic

route(...; ...existing_kw_args..., static_iterator=() -> ...)

This iterator lambda could use for example a database result.