[feat] allow same cbs on different routes

[rook2pawn]

There is an issue where

app.route('/', view)
app.route('/b', view) 
app.route('/c', view)

all end up returning the route as c because wayfarer sticks the route on the reference. which in turn puts the wrong data in state.route as they all resolve to the last known route appended to the callback.

The solution proposed herein simply clones the callback cleanly and thus the trie code doesn't need to be modified. The alternative is to modify trie which is a bit more complex. This is the simplest solution i can think of. Related to #67

@yoshuawuyts @tornqvist

[tornqvist]

This was actually resolved in #66 but never made it to a new release. My later PR (#67) went further and simply removed the route property from the callback, which I think is more robust since a wrapper function can sometimes get in the way if the callback is anything but a function.

[rook2pawn]

Ah, very good! Thanks for the explanation of both.