Allow dynamic mounting with `.use`.

Retains params from the parent application.
Add tests for dynamic mounting.
Remove redundant `.all` function.
Clean up code and syntax issues.
4.0.0-rc3
examples: remove reference to app.router in static-files

closes #1974
Merge pull request #1964 from blakeembrey/modular-routing

keep route params from previous middleware matches
update license year, tired of all the fluff issues about this
remove license text from readme