Add support for inferring return value of user-defined routes

[rpkyle]

As mentioned in https://github.com/plotly/dash-docs/pull/964#discussion_r490372101, users may find it confusing that route handlers added via app$server_route should return TRUE or FALSE depending on whether routr should continue traversing the route stack.

It would be trivial and potentially less confusing to infer TRUE if the function does not return TRUE or FALSE. We should consider adding a check in case users elect to omit TRUE/FALSE, and modifying the Dash for R documentation here once this feature is implemented.

@alexcjohnson @Marc-Andre-Rivet