Add support for default param values

[koenpunt]

Fixes #74

[unitedworx]

How does this work? Default params are for the whole instance of altorouter? Not separately for each route map?

Being able to set default params for each route map could be very useful, would allow more flexibility while using the router since you can e.g easily create an alias for a route while passing the expected parameters via defaults params for this route map

[koenpunt]

We could add it as an extra argument of the map function as well

[mastacontrola]

For what it's worth, I've added this request in my fork just so you don't think it's just been lost of forgotten