Add parsing rules, and a test case, for "Go panicing in a http handler"

[vatine]

Fixes #50.

When doing a test deployment of a fluentd container with this plugin baked in, I tried figuring out the correct configuration by spinning up a Go http server with a URL triggering a panic. It did take me quite a while to figure out why my panic stack traces were not being merged.

This PR adds the capability to merge panics triggered from within a http handler (and a test case).

The example HTTP handler I used to trigger the message is below:

package main

import (
	"net/http"
)

func doPanic(w http.ResponseWriter, r *http.Request) {
	panic("test panic")
}

func main() {
	http.HandleFunc("/", doPanic)
	http.ListenAndServe(":8080", nil)
}

[qingling128]

Sent some minor suggestion. BTW, this one seems to require rebasing.

[qingling128]

LGTM