Return Buffer object with handoff from unique ptr string

[flippmoke]

Added the ability to return the string as a buffer object instead for async methods with this buffer object handing off the result string rather then copying.

Related issue: #69

/cc @springmeyer @GretaCB

[GretaCB]

This is awesome, thanks for pushing this through @flippmoke 🙌

Since this PR adds a new buffer option, could you update the API docs? The skel already has a script that can autogenerate this using documentation.js, and can be run like so:

npm install -g documentation
npm run docs

[flippmoke]

@GretaCB getting Unknown argument: polyglot from https://github.com/mapbox/node-cpp-skel/blob/master/package.json#L14

[springmeyer]

@GretaCB getting Unknown argument: polyglot from https://github.com/mapbox/node-cpp-skel/blob/master/package.json#L14

Per chat, this is because you need to install an older documentation-js version. @GretaCB can you make sure it is documented how to install the old version? (since I presume it is a big lift to deal with trying to get the newer version working and not something we have time for).

[springmeyer]

doc issue ticketed at #150 and worked around in f9fa032