Allow for output Buffer

[osztenkurden]

Hey,

I'm currently writing a data parser, which will work on big files, which have tens / hundreds of thousands of packages packed with snappy. To prevent continuous and repeated allocations I wondered if there would be measurable improvements if I could pass each time the same pre-allocated Buffer, to which this library would write data directly.

I'm open to try and open a PR for it, but I'm extremely new to Rust.