Determine minimum Node version requirement

[TheJaredWilcurt]

Some language features did not exist in older versions of Node, and some dependencies may be using those.

Need to manually test (when the library is in a more stable state) older node versions to see which ones are supported.

There seems to be a big difference between 8.0.0 and 8.3.x. Most of my libraries end up being 8.3+. But would like to at least document the lowest version.

Note: Dependency support for that version matters. devDependencies support does not.