Adding benchmark tests

## Updates

* Add benchmark tests for various decoding inputs

Example benchmark output:

```
$ go test -bench=.*
PASS
Benchmark_Decode	  100000	     26101 ns/op
Benchmark_DecodeBasic	  100000	     22413 ns/op
Benchmark_DecodeEmbedded	  100000	     21879 ns/op
Benchmark_DecodeTypeConversion	   20000	     91155 ns/op
Benchmark_DecodeMap	  100000	     20514 ns/op
Benchmark_DecodeMapOfStruct	   50000	     47987 ns/op
Benchmark_DecodeSlice	  200000	     11588 ns/op
Benchmark_DecodeSliceOfStruct	   50000	     37439 ns/op
Benchmark_DecodeWeaklyTypedInput	  200000	     10502 ns/op
Benchmark_DecodeMetadata	  200000	      8040 ns/op
Benchmark_DecodeMetadataEmbedded	  100000	     23033 ns/op
Benchmark_DecodeTagged	  500000	      7267 ns/op
ok  	github.com/mtchavez/mapstructure	30.543s
```