Workaround for large body

Please note this tool does not works as-is for bodies larger than 64KB. To fix it, add the following lines after https://github.com/ikruglov/slapper/blob/master/slapper.go#L135:
```go
	buf := make([]byte, 0, 64*1024)
	scanner.Buffer(buf, 1024*1024)
```
The second number should be replaced with the maximum length of body you want to use.