Add a .MarshalJSONBuf interface to reduce bytes.Buffer allocations

[pquerna]

Adds a new interface, MarshalJSONBuf which takes a bytes.Buffer as a parameter, instead of returning []bytes.

This is about ~16% faster on my laptop for the goser Log structure.

BenchmarkFFMarshalJSON    200000         10164 ns/op      60.60 MB/s        4227 B/op         46 allocs/op
BenchmarkFFMarshalJSONBuf     200000          8697 ns/op      70.83 MB/s        1959 B/op         42 allocs/op