optional event handling for dynamic router

| Q             | A
| ------------- | ---
| Bug fix?      |  no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #74
| License       | MIT
| Doc PR        | TODO @trsteel88 promised for later this week

optional event handling for dynamic router. replaces #74

/cc @trsteel88