hrbench

add tool similar to `go test -bench . .`

This might be necessary to make #1 workable

```
func HRBenchmarkTimeSleep(b *hrtime.Benchmark) {
    for b.Next() {
        time.Sleep(time.Second)
    }
}
```

```
hrbench run . > a.txt
hrbench histogram .
hrbench plot . > a.svg
hrbench compare *.txt
```