pprof middleware (#138)

Add pprof middleware, enabled by pprof directive.

middleware/pprof/README.md

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+# pprof
+
+pprof publishes runtime profiling data at endpoints under /debug/pprof. You can visit /debug/pprof
+on your site for an index of the available endpoints. By default it will listen on localhost:8053.
+
+> This is a debugging tool. Certain requests (such as collecting execution traces) can be slow. If
+> you use pprof on a live site, consider restricting access or enabling it only temporarily.
+
+For more information, please see [Go's pprof
+documentation](https://golang.org/pkg/net/http/pprof/s://golang.org/pkg/net/http/pprof/) and read
+[Profiling Go Programs](https://blog.golang.org/profiling-go-programs).
+
+## Syntax
+
+~~~
+pprof
+~~~
+
+## Examples
+
+Enable pprof endpoints:
+
+~~~
+pprof
+~~~