Add support for Buffer values

[vecerek]

Fixes #86

The solution is a bit more complex because I don't have access to the Buffer type. If I try to access the type, I get the following error:

Cannot find name 'Buffer'. Do you need to install type definitions for node? Try `npm i --save-dev @types/node`.ts(2580)

This PR implements a Buffer check that does not rely on @types/node to be installed, even though, I would much prefer that solution. The mysql package is built on top of mariadb which is a nodejs-specific implementation. However, I wasn't sure about the platform-specificity of this repo.

@tim-smart any objections against pulling in @types/node? If the packages should be platform-agnostic, I think there's some extra work to do to achieve that for the mysql package, at least.

[tim-smart]

I would like to avoid @types/node if possible, as the sqlite package targets browsers.

Will take a look at this in a couple hours :)