First Class Middleware Support

gorilla/mux now provides first-class support for "middleware" thanks to @roobre! Middleware are handlers that can intercept request & response processing after a route match, but before/after other handlers are invoked. See the documentation for the full details: https://github.com/gorilla/mux#middleware

Previously, developers would have to use a thin wrapper to achieve this per-application. Requests for this functionality built up enough over time such that we've added it to mux's built-in methods.

CHANGELOG:

53c1911 [feat] Add middleware support as discussed in #293 (#294)
5bbbb5b [docs] Add graceful shutdown example (#329)
512169e refactor routeRegexp, particularily newRouteRegexp. (#328)
5ab525f Public test API to set URL params (#322)
7904d2e [docs] Add example usage for Route.HeadersRegexp (#320)
c572efe [docs] Note StrictSlash re-direct behaviour #308 (#321)
65ec724 Create ISSUE_TEMPLATE.md (#318)
4a3d4f3 [bugfix] Fix method subrouter handler matching (#300) (#317)
2d5fef0 [docs] fix outdated UseEncodedPath method docs (#314)