build!: drops Node 6. begin following Node.js LTS schedule

[bcoe]

This skips the EOL Node 8, and moves us to Node 10 as the supported Node.js version for yargs-parser.

Note: we should complete the TypeScript conversion before we release this major version of the library.

TODO:

• I'd like to add a snippet of code that throws if you're on an older Node.js version.

Refs #272

[bcoe]

CC: @QmarkC

[QmarkC]

TODO:

• I'd like to add a snippet of code that throws if you're on an older Node.js version.

Run time or build time e.g. in pretest?
Basic check such as parsing process.version or add a dep like check-node-version?

[bcoe]

@QmarkC I think it's worth adding something along the lines of:

const minNodeVersion = 10;
if (process && process.version) {
  const major = Number(process.version.match(/v([^.]+)/)[1]);
  if (major < minNodeVersion) {
    throw Error(`yargs parser supports a minimum Node.js version of ${minNodeVersion}`)
  }
}