promisified exec and execFile should return a promise with ChildProcess instance attached

[onip]

as per documentation require('child_process').exec and require('child_process').execFile should return a promise with attached a the require('child_process').ChildProcess instance attached to the .child property.

this PR is an adaption of the same code used inside nodejs

> require('child_process').exec[require('util').promisify.custom].toString ()
'(...args) => {\n' +
  '    let resolve;\n' +
  '    let reject;\n' +
  '    const promise = new Promise((res, rej) => {\n' +
  '      resolve = res;\n' +
  '      reject = rej;\n' +
  '    });\n' +
  '\n' +
  '    promise.child = orig(...args, (err, stdout, stderr) => {\n' +
  '      if (err !== null) {\n' +
  '        err.stdout = stdout;\n' +
  '        err.stderr = stderr;\n' +
  '        reject(err);\n' +
  '      } else {\n' +
  '        resolve({ stdout, stderr });\n' +
  '      }\n' +
  '    });\n' +
  '\n' +
  '    return promise;\n' +
  '  }'

[onip]

resolves #881

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[robertsLando]

@onip Could you fix conflicts please? This is ok to merge once conflicts are fixed and tests pass @leerob

[onip]

master merged in

[robertsLando]

LGTM. @leerob merge this when possible please