Added benchmarks, which you can run with `go test -bench .`

Sample output on my laptop using one CPU

```
$ go test -bench .
PASS
BenchmarkMarshallElemDifferentSnippets	  100000	     20018 ns/op
BenchmarkMarshallElemSingleSnippet	  100000	     19688 ns/op
BenchmarkTestMarshallElemNestedSnippet	  100000	     23813 ns/op
BenchmarkMarshallElemMultipleSnippetSameLevel	   50000	     32345 ns/op
BenchmarkMarshallElemSnippetNotFound	  200000	     13220 ns/op
ok  	github.com/fmpwizard/arriba	11.732s

```

the numbers `20018 ns/op` mean that it took about 20 milliseconds to complete one call to the snippet function