Initial benchmark against other libraries

[pelletier]

Compare go-toml to json and yaml. First results:

BenchmarkUnmarshalToml-8             1000  1320117 ns/op  450932 B/op  15072 allocs/op
BenchmarkUnmarshalBurntSushiToml-8   3000   402897 ns/op   82900 B/op   1761 allocs/op
BenchmarkUnmarshalJson-8            20000    66092 ns/op    3536 B/op    101 allocs/op
BenchmarkUnmarshalYaml-8            10000   189600 ns/op   44872 B/op   1058 allocs/op

Plenty of room for improvement.

Related to #167

@bep @kevinburke @moorereason

[bep]

I would also add BurntSushi's TOML to the mix, it seem to be pretty fast.

[kevinburke]

Thats' also not really a fair comparison, the toml library preserves much more data about the underlying file (so it can be accurately reproduced)

[pelletier]

@bep Good idea.
@kevinburke Although that's the case in the current implementation, I don't think Unmarshal should do that, given that this information would be lost anyway.